Hi folks: I am trying to understand ImageSegment methods like activeClasses, swapOutInactiveClasses, discoverActiveClasses among others. One of the comments of those methods says: "NOTE: discoverActiveClasses uses Squeak's ability to detect and recover from faults due to a nil method dictionary. It staches the method dict in with the organization during the time when discovery is in progress (Gag me with a spoon). This is why the faults need to be cleared promptly before resuming normal work with the system. It is also important that classes *do not* refer directly to their method dictionary, but only via the accessor message." So, it seems to HACK (it forces a fault) this MD fault recover to discover inactive classes and other things. Ok, forget that hack for a moment. What questions is, what are these faults? Why a method dictionary can be in nil ? How is this produced ? And how is it fixed? I saw for example this: Behaviour>>methodDict methodDict == nil ifTrue: [self recoverFromMDFaultWithTrace]. ^ methodDict But again...how can that be nil ??? Thank you very much. Marian