I've spent more than 30 minutes looking at a progress bar that said "Time remaining: 0s" on multiple occasions. Oh the joys of (old(er)) application installers.
If only it were merely occasionally...
I've also had UAC pause an installation, but not actually launch its prompt on several occasions.
And that (among other things) is why UAC has die on every Installation. I mean srsly everybody is probably just clicking "ok" anyway, even if there is malware asking permission to delete C: and castrate your dog. Prime example: my sister and her laptop. Occasionally she will tell me that something is wrong and that "there was some popup, but I don't know what it said, I just clicked 'continue' and I dunno lol now things are bad".
Which is why the first thing I do on all of my Windows 7 machines is disable UAC. Useless tat.