Sometimes Mac OS X ignores clicks. The bug is easily reproducible on various applications, including Finder, Safari, Firefox, iTunes.
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I can easily reproduce it with a slower double-click speed setting. Basically, if you manage to get a third click in the same area before the double-click timeout, it ignores subsequent clicks until you wait for the double-click timeout or click somewhere else. I’m not sure if this is a bug or a misguided attempt to ignore superfluous clicks, but it’s annoying behavior. (Edit: it’s probably an attempt to detect triple clicks and such getting in the way, so extra clicks get ignored.)
I find it happens the least if I set the double-click speed one notch left of the fastest setting, but I still manage to trigger it sometimes. If you’re a fast double-clicker, maybe setting it to the fastest setting would work better for you, but then it misses double-clicks if you’re not fast enough.