I'm not sure if my mouse is broken because clicks aren't registering or if I'm just... not hitting things, or there's some other problem? I've been noticing this for a couple of days now. But whenever I deliberately test clicking everything seems fine. 
Update on the mouse: it's definitely broken. The left button is only registering about 50% of clicks now, and it's been driving me crazy. This mouse lasted just barely two years. Time to get a new one, again... 
@kianga I had the same problem only my mouse button broke in two pieces a little bit later.

@Nightraccoon Broke in two!  

Mine usually seem to die in more subtle ways.