Hi Fuel kept throwing strange errors at me the last couple of days. I now found that IdentitySet>>size is not correct for some instances in the image and canât be fixed with #rehash (checked with newest image on newest PharoVM). I noticed that #rehash only replaces âarray' while âtallyâ stays untouched (but âtallyâ holds the wrong sizeâ¦). Notice that all those instances seem to be related to tests (their elements are all Metaclass objects of test classes). Hereâs how to get the (16) corrupted instances: IdentitySet allInstances select: [ :e | e size ~= (e collect: #yourself) size ] Why this causes problems for Fuel: During serialization the IdentitySet size is stored and later its objects. During that step, #do: will evaluate the block less often than it should, based on size (e.g. size is 3, but âarrayâ only holds 2 objects). During materialization all the offsets after an IdentitySet object in the stream are off because of this. FogBugz issue: https://pharo.fogbugz.com/f/cases/13026/IdentitySet-size-reports-wrong-size Cheers, Max