Funkiest for me was a random crash in a C# app. No pattern whatsoever. No function or user role or part of the software or time of day. I had to learn crash dump analysis and bought my first Kindle books (on desktop, no kindle because I needed it asap), one of which had a trick to make a memory issue crash closer to the source, rather than leave it around to be stumbled over hours later. Which was the source of the randomness. Click button, crash. Move mouse, crash.
This had worked perfectly for many years but windows was upgraded underneath it, and some smartass had used clever tricks for a hover menu that didn’t work in a future (safer) version of the OS. A rarely triggered hover menu.
Thank you, authors of advanced windows debugging and advanced .net debugging.