(1) I had to port a game engine to solaris a few years ago. In that engine, if you looked up, you'd see a star emitting light. Guess what 3-letter, all-lowercase identifier was conflicting.

(2) Xt (only useful, really, if you're doing Motif work) had a macro or two that looked exactly like a function call, and had a very natural use case for incrementing an index you feed as an argument. If you knew not to do it, no problem. Otherwise, that was a mean bug to track down.