Execution of an Action on a large number of files can get stuck (during which it ignores Cancel clicks) progressing only while the mouse is moved or a key is held.
E.g. On 662 files I run an empty Action group. The progress dialog runs until the bar is at about 10% and then stops, the text saying "0 of 662 tags formatted". If I do nothing, the bar remains stationary indefinitely. If I move the mouse or hold down SHIFT, the bar moves on, and if I keep the mouse moving or SHIFT down, the operation completes in good time.
Reboot doesn't fix this. Likewise app reinstall. I've tried it only on one machine (WinXP SP2).