Hello, I found method doesNotUnderstand have very strange implementation Object>>doesNotUnderstand: aMessage | exception resumeValue | (exception := MessageNotUnderstood new) message: aMessage; receiver: self. resumeValue := exception signal. ^exception reachedDefaultHandler ifTrue: [aMessage sentTo: self] ifFalse: [resumeValue] When Error has no handlers #defaultAction is executed. For MessageNotUnderstood #defaultAction is UnhandledError signal. And for UnhandledError #defaultAction is opening debugger. If you change it to do nothing (^self) you get infinite loop on any MessageNotUnderstood signal. It is because "ifTrue" branch executed in doesNotUnderstood method and message resend to receiver. Why method implemented such way? Why it is differ from ProtoObject implementation. I think It's bug implementation. I found this problem when I try implement stuff to turn off openning debugger on unhandled error (and write it to log) UnhandledError>>defaultAction "The current computation is terminated. The cause of the error should be logged or reported to the user. If the program is operating in an interactive debugging environment the computation should be suspended and the debugger activated." ^ToolSet debugError: exception. ToolSet class>>debugError: anError "Handle an otherwise unhandled error" self default ifNil:[ | ctx | Smalltalk logError: anError description inContext: (ctx := anError signalerContext) to: 'PharoDebug.log'. self inform: (anError description, String cr, ctx shortStack). ^anError return]. ^self default debugError: anError I just implement subclass of StandartToolSet with empty method #debugError:. Then I do "ToolSet default: MyToolSet". And now unhandled errors raise infinit loop and never return. I fix that by: MyToolSet>>debugError: anError anError class = MessageNotUnderstand ifTrue: [anError reachedDefaultHandler: true]. I think this is hack. But it is work. No errors show to user.And all work good. What do you think? Best regards, Denis