It used to be that pressing the space bar in Quicktime Player would pause a video. However, beginning with Lion, it seems that it sometimes pauses the video, but other times it skips or jumps the video to a random location.
The problem is if you use the two fingered gesture to fast-forward or rewind the video. As soon as you do, you'll notice that the tab focus jumps to the position indicator.
Here's a normal Quicktime Player control before using gestures:
Here's what the control looks like as soon as you use a fast-forward or rewind gesture:
Notice that the diamond position indicator is now blue.
Seems to be a particularly annoying bug in the player.