I somewhat agree that buttonless mouse gestures could be accidentally generated. Here's how PopMouse solved the two problems Graham brought up:
1. PopMouse used a stricter criteria to determine whether the action was accidental or not. For the gesture "O", it had to be a minimum size and a minimum curvature. So I couldn't draw small circles nor extremely stretched ovals. It also had a time limit so I had to draw them relatively fast.
2. PopMouse only supported 4 gestures; clockwise O, anti-clockwise O, right-first wiggle and left-first wiggle. There weren't any confusions in these gestures.
I personally only used the "O" gestures to go back and forward on the browser so the risks of accidentally performing a gesture are small. I could always swing the mouse back to reverse the action!