Hi, I found this comment in TBehavior>>indexIfCompact : "Verify if the compactClassesArray and indexIfCompact are coheren Smalltalk compactClassesArray doWithIndex: [:c :i | c == nil ifFalse: [c indexIfCompact = i ifFalse: [self halt]]] " Since it *does* halt, I rewrote to: | inconsistencies | inconsistencies := OrderedCollection new. Smalltalk compactClassesArray doWithIndex: [ :class :index | class ifNotNil: [ class indexIfCompact = index ifFalse: [ inconsistencies add: { class. index. class indexIfCompact } ]]]. inconsistencies."---> an OrderedCollection({LargeNegativeInteger. 4. 0} {LargePositiveInteger. 5. 0})" I checked that in Pharo 2 they are consistent, so I guess this should be fixed. I ignore deep vm internals so... how would you fix this? I tried to #becomeUncompact to then #becomeCompact again, but it didn't work out-of-the-box. Cheers, MartÃn