I've started taking a look at the problem reported by Michael Roberts. I got slightly different misbehavior using the 10259-ClosureBootstrap download on linux than reported by Michael, but in checking things out I discovered that the code in the Debugger for the *Core* downloads is different from the *dev* downloads. For example the source for Debugger>>selectedMessageName in the *Core* image (10259-ClosureBootstrap) is: selectedMessageName "Answer the message selector of the currently selected context. If the method is unbound we can still usefully answer its old selector." | selector | selector := self selectedContext methodSelector. ^(selector ~~ self selectedContext method selector and: [selector beginsWith: 'DoIt']) ifTrue: [self selectedContext method selector] ifFalse: [selector] and I get a DNU for #methodSelector. On the other hand in the *dev* image (pharo0.1-10259dev09.03.3) the source is: selectedMessageName "Answer the message selector of the currently selected context." ^self selectedContext selector When I looked at the Pharo0.1Core-10259 image, the source is identical to the pharo0.1-10259dev09.03.3, so the Closure changeset is apparently introducing some 'unwanted' changes to the Debugger (and perhaps elsewhere?). Dale