Just to update, I've concluded that it's my mouse. (Can't edit previous post.)
Seems to be a problem with Razer mice parts sticking to the buttons while clicking, affecting several models. Quite annoying. A fix, if anyone else experiences this is to either open up the mouse and cleaning it with a Q-tip and cleaning solution, or simply blow or use Dust Off into the button cracks.
Feel free to close this thread.