Interactively decompiling bytecode to see if a refactoring is safe! Smalltalk guru meditation: https://www.mirandabanda.org/cogblog/2017/02/07/smalltalk-scanning-and-shcontrol-structures/
miniblog.
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Decompiling Super Mario 64 by carefully writing C that produces the same assembly!
https://gbatemp.net/threads/super-mario-64-has-been-decompiled.542918/
A huge task, although shipping the game without aggressive optimisations made it easier apparently.