There's several bugs from some 8+ years ago that I remember fairly well, because they were truly interesting.

One I didn't work on, but was the sort of stuff legends are made of. A very specific revision of Blink that was never shipped as a Chrome release but got shipped as BlackberryOS's default browser had a bug around one of the bitshift operators (can't recall which), which would give a wrong result around once in a million operations. The desktop build of that particular revision would only trigger that bug semi-reliably, but the Blackberry build would trigger it deterministically. Bitshifts are not particularly common in most JS codebases, but this was a bitcoin wallet, and bitshifts are used by the thousands as part of e.g. key derivations. "Key derivation" is exactly where this bug triggered for a user, and he lost a whole bunch of BTC to that bug. We were almost 100% sure the guy was trying to defraud us with that bug report but something made us itchy and we had somebody keep digging until we found the bug. The good news is that, because the Blackberry version was deterministic, we managed to reproduce the bug to the point of recreating the badly-generated keys and successfully recovered the guy's BTC.

One I worked on myself, diagnosing it involved attaching both IntelliJ's debugger and GDB to a running Java application, so I could set breakpoints both in Java land and in the C++/JNI component of a JDBC driver. My colleague tracked it down to an NPE at that boundary, and was stuck on how to proceed from there.

A fun one from a couple of years involved a script that ran fine on Firefox but failed on Chrome. This was around the time when fat arrow notation was introduced for JS functions. Introducing that involved changes to the parser, of course, and a vendor's minified scripts triggered a bug in the Chrome's JS parser such that it was seeing a fat arrow where there wasn't one. We solved that one by re-minifying the vendor's minified script with a different minifier that wouldn't generate the offending pattern.