The interviewers turned up late, walking right past me to set themselves up without even bothering to say hello.
Two interviewers, one of whom very clearly did not want to be there. Part way through, said interviewer began talking to her colleague about something completely irrelevant, leaving me sat there unsure what to do.
I was told I would be given a follow-up call the next week, and I would know what was happening with the job as soon as a decision had been made.
After two weeks of no contact, I emailed the recruiter to ask if any decisions had been made or if the interviews were still ongoing. I received a reply (a very informal email via mobile) informing me that someone else had been chosen - and it was very obvious that person had been chosen days, if not weeks earlier!
Absolutely appalling way to treat people, especially when you've given assurances they'd be informed of the outcome. It was very clear I'd have received no information had I not emailed myself.