Around 10 years ago I found a JIT bug in a JDK from a big Java vendor. A new version of a web application server had been applied. The production application crashed after around 30 minutes of running, almost simultaneously on both production sites. It was an internal checksum calculation in the application that failed - an obscure error never seen before. The upgrade was rolled back immediately. I was assigned to the case and of course didn't suspect a JIT error. But within a week of investigation I started suspecting it must be (but I didn't dare tell anyone!) and I eventually managed to show this and reproduce it consistently. The vendor confirmed and made a temporary workaround via switches that disabled some new optimizations. Later a real fix was shipped.

I've also found 3-4 JavaScript JIT compiler errors in major browsers, all confirmed. I was a developer on what was for its time a quite complicated JavaScript solution, so we tended to encounter obscure JavaScript errors before others.