The mistake I seem to make most often -- especially when "patching" or "enhancing" code that was already running -- is forgetting to put the semi-colon at the end of a newly-added line. (Or forgetting the closing slash/bracket on a regex, or...)

That sort of thing always makes a compile-time error, and the error report always sites a line number somewhere later than the offending line. Drives me up a wall -- I should be habituated to typing ";" before the "return" key or whatever, but I still forget...

Since I use emacs, a forgetten close-bracket usually causes unexpected behavior with auto-indenting, so I catch it while editing, but the forgotten slash or semi-colon seems to squeak through fairly often.