I often experience this with mireless mice, stylus/tablet devices or with users that have less motor coordination.

1. Click a combobox
2. The combobox contents flash by, and the combobox "closes".
   You have to try again.

What happened is that during the click, the mouse cursor moved slightly, and combobox thought it was a click-drag-release.

Instead, the drag should be ignored if the release happened within less than 200-500 ms, because a release that occurs that fast is pretty sure to be a simple click with jitter.

I can also note that this same phenomenon applies to popup menus (ex: when right-clicking). If I release my right mouse button within a split second, you can be pretty sure it was because I just wanted to show the menu, and that I did not have time to make a choice yet.