Found it! I was accidentally writing a class as the method class literal into a compiled method. This is usually an association (or `ClassBinding`). This explains the segfault upon `super` send.
The reason this only showed up in trunk is that i had a correct `Environment>>associationAt:ifAbsent:` but not for `#bindingOf:ifAbsent:`, which, in trunk, is the new message send in `Class>>binding`.
I'm closing this issue. Here's an MWE in case you want to do something with the segfault:
```smalltalk | class method | Object subclass: #Foo instanceVariableNames: 'env' classVariableNames: '' poolDictionaries: '' category: 'Bar'.
class := Smalltalk at: #Foo. class compile: 'printString super prinstString'.
method := (Smalltalk at: #Foo) compiledMethodAt: #printString. method literalAt: method numLiterals put: class. class new printString ```