I caught a JIT bug in .NET back when I worked at Microsoft. Unfortunately I don't remember the specifics, but it was doing something wrong when trying to optimize a bit of code using AVX instructions.

I was immediately sure it was a compiler bug when I realized I could make the code work correctly if I changed the order of variable declarations. It would happen on something like:

int a; int b; float c;

(again, that's only illustrative - too long ago for me to remember the specifics)

But not with:

int a; float c; int b;

Folks in my team didn't want to believe it at first :)

Edit: found it here - https://github.com/dotnet/runtime/issues/17395

Not quite as I described but close.