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This is a good list! Another fun quirk: because x86 is a register-memory architecture and allows all kinds of variants of reg/mem operand encodings, there are a handful of equivalent encodings with exactly the same lengths (and just slightly different ModR/M bytes). You can take advantage of this to do software fingerprinting or, in my case, steganography without changing an executable’s size or semantics[1].
[1]: https://github.com/woodruffw/steg86
I'd guess it's "undocumented", not "undefined". Don't know how the situation is on x86, but on the Z80 there were indeed some slight differences in undocumented behaviour depending on the CPU vendor, but those are so obscure that it hardly affected any real world code, because it affected the undocumented flag bits 3 and 5, and this behaviour was only properly decoded in the 2000's (https://github.com/floooh/emu-info/blob/master/z80/memptr_en...).
The only CPU I know with actual undefined behaviour is the 6502 for some of the undocumented/illegal opcodes which can yield different results based on things like current CPU temperature (see the ANE/XAA instruction description: https://www.masswerk.at/nowgobang/2021/6502-illegal-opcodes)