While viewing photos using the Preview window, deleting photos too quickly causes a problem. I'm using the Opt-backspace key combo to delete them without a confirmation dialog and if you delete several files consecutively and do it too quickly, the progress bar window comes up and won't go away. The Abort button does nothing. Although I can continue to select different images in the contact sheet, PM then stops redrawing the sheet or Preview window. I have to Quit (but not Force Quit) PM to progress.
If the frontmost window is the contact sheet, it seems to work OK. If the frontmost window is the Preview window (I have two monitors so have the preview and contact sheet visible at the same time) I can repeat this effect.
I'm using PM 5.0, build 13158 (7de0ba6), on OSX 10.7.4.