probably the big one for me is web video players. they're basically antagonistic to the user.
OK, so I've clicked it and the video started playing. clicking it again should pause it, right? nope, now it's fullscreen. click the "un-fullscreen" button and click the little pause button.
now let's say I want to scrub back to a particular part of the video. as soon as you let go of the scrubber thing, the video un-pauses regardless of whether you were ready to start watching. same with the mute/unmute buttons un-pausing again for no apparent reason. so aggravating.
obviously they're not all like this (youtube is ok) but the twitter/facebook/etc. ones are just abysmal...