The most common (and annoying) phenomenon for me is the smart-ass 'non-conversations' that can happily continue the entire evening. Guys are the main culprits - basically, just a succession of slagging & innuendo together with the relating of hilarious tales of the previous night's slagging and innuendo. There's usually some hapless slightly geekish victim within the group too who bears the brunt of much of this.
They actually don't say anything at all, all night.
Ask them the next morning and they "had a feckin' great night"