⌥+space instead of just
space is an easy mistake to make, especially if you just used
⌥ for something else.
In Cocoa text fields,
⌥+space inserts a typographical em space, which e.g. Ruby's parser does not treat as a regular space. Painful debugging ensues. Even after being bitten by this several times, it still gets me now and then.
I finally got around to fixing it: the Prevent Em Space TextMate command slips into the Source bundle and simply outputs a regular space if you press