(I've complained about some of these on HN before, but a particulary bad offender is that blog theme that lets mobile users swipe left or right to go to the previous or next blog post. Aside from being undiscoverable, ugly (since the swiping entirely misses the point of iOS's UI, acting only after a swipe has been completed rather than smoothly tracking the finger with an interface element-- Google Images also has this problem) and useless (who wants to skip to the previous or next blog entry in the middle of reading the current one?), this feature always catches my not-quite-vertical swipes and tries to take me off the page while I'm reading!
Of course this doesn't have anything to do with Hacker News, but I think it's the mindset that adding a mobile site is solving a compatibility issue, thus always an improvement over inaction, that's responsible for the vast number of crappy mobile sites out there.)