I've made a small modification to t/pmc/freeze_25.pir, that is I added a
`say $S0` statement on line 12 (between the freeze and thaw statements) and ran
the following command 30 times consecutively with one second sleep between each
invocation :

perl -e 'print(time(),"\n")'; ./parrot t/pmc/freeze_25.pir

Standard output:

1:1231282272
2:1 2 3 6
3:þPBC
4:
5:1]OPTableparrot;OPTable
6:                       <(ÿÿÿÿÿÿ!barP!foobar!bar"!foobar&((ÿÿÿ,<24(ÿÿÿ:ÿÿÿ<<init@2Finitinitinit!parrotinitDR!OPTableinitHFdinitinitinit!parrotÿÿÿÿinittestNdtesttesttest!parrotÿÿÿÿtestPR!OPTableV!OPTable%!keyX(ÿÿÿ%!klen^ÿÿÿ&!wsbÿÿÿ1namedR!&!ws1namej!%!klen1namen!%!key
7:^[[?1;2c^[[?1;2c1 2 3 6


line 1 is the time returned by perl.
line 6 and line 7 are the only one changing.

I compiled the output of the 30 calls:
'GOOD' means the test passed successfully while 'BAD' triggered the NULL PMC error.


BAD  1231282271 <(ÿÿÿÿÿÿ!barP!foobar!bar"!foobar&((ÿÿÿ,<24(ÿÿÿ:ÿÿÿ<<init@2Finitinitinit!parrotinitDR!OPTableinitHFdinitinitinit!parrotÿÿÿÿinittestNdtesttesttest!parrotÿÿÿÿtestPR!OPTableV!OPTable&!wsX(ÿÿÿ%!key^ÿÿÿ%!klenbÿÿÿ1namedR!%!klen1namej!%!key1namen!&!ws
GOOD 1231282272 <(ÿÿÿÿÿÿ!barP!foobar!bar"!foobar&((ÿÿÿ,<24(ÿÿÿ:ÿÿÿ<<init@2Finitinitinit!parrotinitDR!OPTableinitHFdinitinitinit!parrotÿÿÿÿinittestNdtesttesttest!parrotÿÿÿÿtestPR!OPTableV!OPTable%!keyX(ÿÿÿ%!klen^ÿÿÿ&!wsbÿÿÿ1namedR!&!ws1namej!%!klen1namen!%!key
GOOD 1231282273 <(ÿÿÿÿÿÿ!foobarP!bar!foobar"!bar&((ÿÿÿ,<24(ÿÿÿ:ÿÿÿ<<init@2Finitinitinit!parrotinitDR!OPTabletestHdtesttesttest!parrotÿÿÿÿtestinitNFdinitinitinit!parrotÿÿÿÿinitPR!OPTableV!OPTable%!keyX(ÿÿÿ%!klen^ÿÿÿ&!wsbÿÿÿ1namedR!&!ws1namej!%!klen1namen!%!key
GOOD 1231282274 <(ÿÿÿÿÿÿ!barP!foobar!bar"!foobar&((ÿÿÿ,<24(ÿÿÿ:ÿÿÿ<<init@2Finitinitinit!parrotinitDR!OPTabletestHdtesttesttest!parrotÿÿÿÿtestinitNFdinitinitinit!parrotÿÿÿÿinitPR!OPTableV!OPTable%!keyX(ÿÿÿ%!klen^ÿÿÿ&!wsbÿÿÿ1namedR!&!ws1namej!%!klen1namen!%!key
GOOD 1231282275 <(ÿÿÿÿÿÿ!barP!foobar!bar"!foobar&((ÿÿÿ,<24(ÿÿÿ:ÿÿÿ<<init@2Finitinitinit!parrotinitDR!OPTabletestHdtesttesttest!parrotÿÿÿÿtestinitNFdinitinitinit!parrotÿÿÿÿinitPR!OPTableV!OPTable%!keyX(ÿÿÿ%!klen^ÿÿÿ&!wsbÿÿÿ1namedR!&!ws1namej!%!klen1namen!%!key
GOOD 1231282277 <(ÿÿÿÿÿÿ!barP!foobar!bar"!foobar&((ÿÿÿ,<24(ÿÿÿ:ÿÿÿ<<init@2Finitinitinit!parrotinitDR!OPTableinitHFdinitinitinit!parrotÿÿÿÿinittestNdtesttesttest!parrotÿÿÿÿtestPR!OPTableV!OPTable%!keyX(ÿÿÿ%!klen^ÿÿÿ&!wsbÿÿÿ1namedR!&!ws1namej!%!klen1namen!%!key
GOOD 1231282278 <(ÿÿÿÿÿÿ!foobarP!bar!foobar"!bar&((ÿÿÿ,<24(ÿÿÿ:ÿÿÿ<<init@2Finitinitinit!parrotinitDR!OPTabletestHdtesttesttest!parrotÿÿÿÿtestinitNFdinitinitinit!parrotÿÿÿÿinitPR!OPTableV!OPTable%!keyX(ÿÿÿ%!klen^ÿÿÿ&!wsbÿÿÿ1namedR!&!ws1namej!%!klen1namen!%!key
GOOD 1231282279 <(ÿÿÿÿÿÿ!foobarP!bar!foobar"!bar&((ÿÿÿ,<24(ÿÿÿ:ÿÿÿ<<init@2Finitinitinit!parrotinitDR!OPTabletestHdtesttesttest!parrotÿÿÿÿtestinitNFdinitinitinit!parrotÿÿÿÿinitPR!OPTableV!OPTable%!keyX(ÿÿÿ%!klen^ÿÿÿ&!wsbÿÿÿ1namedR!&!ws1namej!%!klen1namen!%!key
GOOD 1231282280 <(ÿÿÿÿÿÿ!barP!foobar!bar"!foobar&((ÿÿÿ,<24(ÿÿÿ:ÿÿÿ<<init@2Finitinitinit!parrotinitDR!OPTabletestHdtesttesttest!parrotÿÿÿÿtestinitNFdinitinitinit!parrotÿÿÿÿinitPR!OPTableV!OPTable%!keyX(ÿÿÿ%!klen^ÿÿÿ&!wsbÿÿÿ1namedR!&!ws1namej!%!klen1namen!%!key
GOOD 1231282281 <(ÿÿÿÿÿÿ!barP!foobar!bar"!foobar&((ÿÿÿ,<24(ÿÿÿ:ÿÿÿ<<init@2Finitinitinit!parrotinitDR!OPTabletestHdtesttesttest!parrotÿÿÿÿtestinitNFdinitinitinit!parrotÿÿÿÿinitPR!OPTableV!OPTable%!keyX(ÿÿÿ%!klen^ÿÿÿ&!wsbÿÿÿ1namedR!&!ws1namej!%!klen1namen!%!key
GOOD 1231282282 <(ÿÿÿÿÿÿ!barP!foobar!bar"!foobar&((ÿÿÿ,<24(ÿÿÿ:ÿÿÿ<<init@2Finitinitinit!parrotinitDR!OPTableinitHFdinitinitinit!parrotÿÿÿÿinittestNdtesttesttest!parrotÿÿÿÿtestPR!OPTableV!OPTable%!keyX(ÿÿÿ%!klen^ÿÿÿ&!wsbÿÿÿ1namedR!&!ws1namej!%!klen1namen!%!key
GOOD 1231282283 <(ÿÿÿÿÿÿ!barP!foobar!bar"!foobar&((ÿÿÿ,<24(ÿÿÿ:ÿÿÿ<<init@2Finitinitinit!parrotinitDR!OPTabletestHdtesttesttest!parrotÿÿÿÿtestinitNFdinitinitinit!parrotÿÿÿÿinitPR!OPTableV!OPTable%!keyX(ÿÿÿ%!klen^ÿÿÿ&!wsbÿÿÿ1namedR!&!ws1namej!%!klen1namen!%!key
GOOD 1231282284 <(ÿÿÿÿÿÿ!foobarP!bar!foobar"!bar&((ÿÿÿ,<24(ÿÿÿ:ÿÿÿ<<init@2Finitinitinit!parrotinitDR!OPTabletestHdtesttesttest!parrotÿÿÿÿtestinitNFdinitinitinit!parrotÿÿÿÿinitPR!OPTableV!OPTable%!keyX(ÿÿÿ%!klen^ÿÿÿ&!wsbÿÿÿ1namedR!&!ws1namej!%!klen1namen!%!key
GOOD 1231282285 <(ÿÿÿÿÿÿ!barP!foobar!bar"!foobar&((ÿÿÿ,<24(ÿÿÿ:ÿÿÿ<<init@2Finitinitinit!parrotinitDR!OPTabletestHdtesttesttest!parrotÿÿÿÿtestinitNFdinitinitinit!parrotÿÿÿÿinitPR!OPTableV!OPTable%!keyX(ÿÿÿ%!klen^ÿÿÿ&!wsbÿÿÿ1namedR!&!ws1namej!%!klen1namen!%!key
GOOD 1231282286 <(ÿÿÿÿÿÿ!barP!foobar!bar"!foobar&((ÿÿÿ,<24(ÿÿÿ:ÿÿÿ<<init@2Finitinitinit!parrotinitDR!OPTabletestHdtesttesttest!parrotÿÿÿÿtestinitNFdinitinitinit!parrotÿÿÿÿinitPR!OPTableV!OPTable%!keyX(ÿÿÿ%!klen^ÿÿÿ&!wsbÿÿÿ1namedR!&!ws1namej!%!klen1namen!%!key
BAD  1231282287 <(ÿÿÿÿÿÿ!barP!foobar!bar"!foobar&((ÿÿÿ,<24(ÿÿÿ:ÿÿÿ<<init@2Finitinitinit!parrotinitDR!OPTableinitHFdinitinitinit!parrotÿÿÿÿinittestNdtesttesttest!parrotÿÿÿÿtestPR!OPTableV!OPTable&!wsX(ÿÿÿ%!key^ÿÿÿ%!klenbÿÿÿ1namedR!%!klen1namej!%!key1namen!&!ws
GOOD 1231282288 <(ÿÿÿÿÿÿ!barP!foobar!bar"!foobar&((ÿÿÿ,<24(ÿÿÿ:ÿÿÿ<<init@2Finitinitinit!parrotinitDR!OPTableinitHFdinitinitinit!parrotÿÿÿÿinittestNdtesttesttest!parrotÿÿÿÿtestPR!OPTableV!OPTable%!keyX(ÿÿÿ%!klen^ÿÿÿ&!wsbÿÿÿ1namedR!&!ws1namej!%!klen1namen!%!key
GOOD 1231282289 <(ÿÿÿÿÿÿ!foobarP!bar!foobar"!bar&((ÿÿÿ,<24(ÿÿÿ:ÿÿÿ<<init@2Finitinitinit!parrotinitDR!OPTabletestHdtesttesttest!parrotÿÿÿÿtestinitNFdinitinitinit!parrotÿÿÿÿinitPR!OPTableV!OPTable%!keyX(ÿÿÿ%!klen^ÿÿÿ&!wsbÿÿÿ1namedR!&!ws1namej!%!klen1namen!%!key
GOOD 1231282290 <(ÿÿÿÿÿÿ!barP!foobar!bar"!foobar&((ÿÿÿ,<24(ÿÿÿ:ÿÿÿ<<init@2Finitinitinit!parrotinitDR!OPTabletestHdtesttesttest!parrotÿÿÿÿtestinitNFdinitinitinit!parrotÿÿÿÿinitPR!OPTableV!OPTable%!keyX(ÿÿÿ%!klen^ÿÿÿ&!wsbÿÿÿ1namedR!&!ws1namej!%!klen1namen!%!key
GOOD 1231282291 <(ÿÿÿÿÿÿ!barP!foobar!bar"!foobar&((ÿÿÿ,<24(ÿÿÿ:ÿÿÿ<<init@2Finitinitinit!parrotinitDR!OPTabletestHdtesttesttest!parrotÿÿÿÿtestinitNFdinitinitinit!parrotÿÿÿÿinitPR!OPTableV!OPTable%!keyX(ÿÿÿ%!klen^ÿÿÿ&!wsbÿÿÿ1namedR!&!ws1namej!%!klen1namen!%!key
BAD  1231282292 <(ÿÿÿÿÿÿ!barP!foobar!bar"!foobar&((ÿÿÿ,<24(ÿÿÿ:ÿÿÿ<<init@2Finitinitinit!parrotinitDR!OPTabletestHdtesttesttest!parrotÿÿÿÿtestinitNFdinitinitinit!parrotÿÿÿÿinitPR!OPTableV!OPTable%!klenX(ÿÿÿ&!ws^ÿÿÿ%!keybÿÿÿ1namedR!%!key1namej!&!ws1namen!%!klen
GOOD 1231282293 <(ÿÿÿÿÿÿ!barP!foobar!bar"!foobar&((ÿÿÿ,<24(ÿÿÿ:ÿÿÿ<<init@2Finitinitinit!parrotinitDR!OPTableinitHFdinitinitinit!parrotÿÿÿÿinittestNdtesttesttest!parrotÿÿÿÿtestPR!OPTableV!OPTable%!keyX(ÿÿÿ%!klen^ÿÿÿ&!wsbÿÿÿ1namedR!&!ws1namej!%!klen1namen!%!key
GOOD 1231282294 <(ÿÿÿÿÿÿ!foobarP!bar!foobar"!bar&((ÿÿÿ,<24(ÿÿÿ:ÿÿÿ<<init@2Finitinitinit!parrotinitDR!OPTabletestHdtesttesttest!parrotÿÿÿÿtestinitNFdinitinitinit!parrotÿÿÿÿinitPR!OPTableV!OPTable%!keyX(ÿÿÿ%!klen^ÿÿÿ&!wsbÿÿÿ1namedR!&!ws1namej!%!klen1namen!%!key
GOOD 1231282295 <(ÿÿÿÿÿÿ!foobarP!bar!foobar"!bar&((ÿÿÿ,<24(ÿÿÿ:ÿÿÿ<<init@2Finitinitinit!parrotinitDR!OPTabletestHdtesttesttest!parrotÿÿÿÿtestinitNFdinitinitinit!parrotÿÿÿÿinitPR!OPTableV!OPTable%!keyX(ÿÿÿ%!klen^ÿÿÿ&!wsbÿÿÿ1namedR!&!ws1namej!%!klen1namen!%!key
GOOD 1231282296 <(ÿÿÿÿÿÿ!barP!foobar!bar"!foobar&((ÿÿÿ,<24(ÿÿÿ:ÿÿÿ<<init@2Finitinitinit!parrotinitDR!OPTabletestHdtesttesttest!parrotÿÿÿÿtestinitNFdinitinitinit!parrotÿÿÿÿinitPR!OPTableV!OPTable%!keyX(ÿÿÿ%!klen^ÿÿÿ&!wsbÿÿÿ1namedR!&!ws1namej!%!klen1namen!%!key
GOOD 1231282297 <(ÿÿÿÿÿÿ!barP!foobar!bar"!foobar&((ÿÿÿ,<24(ÿÿÿ:ÿÿÿ<<init@2Finitinitinit!parrotinitDR!OPTabletestHdtesttesttest!parrotÿÿÿÿtestinitNFdinitinitinit!parrotÿÿÿÿinitPR!OPTableV!OPTable%!keyX(ÿÿÿ%!klen^ÿÿÿ&!wsbÿÿÿ1namedR!&!ws1namej!%!klen1namen!%!key
GOOD 1231282298 <(ÿÿÿÿÿÿ!barP!foobar!bar"!foobar&((ÿÿÿ,<24(ÿÿÿ:ÿÿÿ<<init@2Finitinitinit!parrotinitDR!OPTableinitHFdinitinitinit!parrotÿÿÿÿinittestNdtesttesttest!parrotÿÿÿÿtestPR!OPTableV!OPTable%!keyX(ÿÿÿ%!klen^ÿÿÿ&!wsbÿÿÿ1namedR!&!ws1namej!%!klen1namen!%!key
GOOD 1231282299 <(ÿÿÿÿÿÿ!barP!foobar!bar"!foobar&((ÿÿÿ,<24(ÿÿÿ:ÿÿÿ<<init@2Finitinitinit!parrotinitDR!OPTabletestHdtesttesttest!parrotÿÿÿÿtestinitNFdinitinitinit!parrotÿÿÿÿinitPR!OPTableV!OPTable%!keyX(ÿÿÿ%!klen^ÿÿÿ&!wsbÿÿÿ1namedR!&!ws1namej!%!klen1namen!%!key
GOOD 1231282301 <(ÿÿÿÿÿÿ!barP!foobar!bar"!foobar&((ÿÿÿ,<24(ÿÿÿ:ÿÿÿ<<init@2Finitinitinit!parrotinitDR!OPTabletestHdtesttesttest!parrotÿÿÿÿtestinitNFdinitinitinit!parrotÿÿÿÿinitPR!OPTableV!OPTable%!keyX(ÿÿÿ%!klen^ÿÿÿ&!wsbÿÿÿ1namedR!&!ws1namej!%!klen1namen!%!key
GOOD 1231282302 <(ÿÿÿÿÿÿ!barP!foobar!bar"!foobar&((ÿÿÿ,<24(ÿÿÿ:ÿÿÿ<<init@2Finitinitinit!parrotinitDR!OPTabletestHdtesttesttest!parrotÿÿÿÿtestinitNFdinitinitinit!parrotÿÿÿÿinitPR!OPTableV!OPTable%!keyX(ÿÿÿ%!klen^ÿÿÿ&!wsbÿÿÿ1namedR!&!ws1namej!%!klen1namen!%!key
                                                                                                                                                                               ^----------------------------------------------------------------------------------^
For comparison and clarity, here are only the 3 'BAD' lines
BAD  1231282271 <(ÿÿÿÿÿÿ!barP!foobar!bar"!foobar&((ÿÿÿ,<24(ÿÿÿ:ÿÿÿ<<init@2Finitinitinit!parrotinitDR!OPTableinitHFdinitinitinit!parrotÿÿÿÿinittestNdtesttesttest!parrotÿÿÿÿtestPR!OPTableV!OPTable&!wsX(ÿÿÿ%!key^ÿÿÿ%!klenbÿÿÿ1namedR!%!klen1namej!%!key1namen!&!ws
BAD  1231282287 <(ÿÿÿÿÿÿ!barP!foobar!bar"!foobar&((ÿÿÿ,<24(ÿÿÿ:ÿÿÿ<<init@2Finitinitinit!parrotinitDR!OPTableinitHFdinitinitinit!parrotÿÿÿÿinittestNdtesttesttest!parrotÿÿÿÿtestPR!OPTableV!OPTable&!wsX(ÿÿÿ%!key^ÿÿÿ%!klenbÿÿÿ1namedR!%!klen1namej!%!key1namen!&!ws
BAD  1231282292 <(ÿÿÿÿÿÿ!barP!foobar!bar"!foobar&((ÿÿÿ,<24(ÿÿÿ:ÿÿÿ<<init@2Finitinitinit!parrotinitDR!OPTabletestHdtesttesttest!parrotÿÿÿÿtestinitNFdinitinitinit!parrotÿÿÿÿinitPR!OPTableV!OPTable%!klenX(ÿÿÿ&!ws^ÿÿÿ%!keybÿÿÿ1namedR!%!key1namej!&!ws1namen!%!klen

I have no idea whether it is relevant or not, but it seems that all the 'GOOD'
lines end up in a very similar fashion. I hope this helps.

G.