Whats the most troubling syntax & logical error you ever encountered

Answer 1

Check Brainf*ck "programming language".

Answer 2

I've never had a troubling syntax errors. While my logic errors are few and far between them, nobody has ever found them for me despite many times asking for help. To make matters worst, I tend to see what I meant to code, not what I actually coded, so, when I screw up, it takes me days to find.

The last one I remember was a timing issue. The device driver for the Ethernet hardware had an undocumented issue that conflicted with my protocol usage of it. I eventually patched the fix into the driver, after 4 weeks of debugging random problems.