The early-to-mid-90s "High C/C++" compiler had a bug in its floating point library for basic math functions. It ended up being a bit of a Heisenbug to track down, and I didn't initially believe it wasn't my code, but it actually ended up being in their supplied library.
It took me maybe three days to track down, from first clues to final resolution, on a 486/50 luggable with the orange on black monochrome built-in screen.