I've tanked just about every interview I've ever had. In fact, I think the one good interview I had was completely sabotaged when unbeknownst to me my former boss who volunteered to serve as a reference decided to make shit up because I didn't take my old job back when it was offered to me a few weeks after I was originally downsized!
Yeah, that probably cost me about 8 months of work, but on the bright side I didn't end up testing Sony HDTVs for a living, which is incredibly boring.
As for coding questions in interviews. They're usually pretty ridiculous. Write an extension method? Give me a break. That's exactly the sort of thing you would Google when you need it. Luckily I interviewed with someone who wanted someone with broad programming skills instead of being able to recite MSDN documentation by heart.
I mean, I guess I understand that you want to be confident that a candidate has the skills they claim... but I got way too many questions in various interviews where anybody who isn't an OCD mouth-breather would just figure out the right answer with 2 seconds of googling or intellisense spamming. No, I can't write a fucking event handler for random mouse/keyboard input without looking up what event arguments I get sent. So the fuck what?!? I've seriously gone on interviews where the fucker was mad that I didn't know some of the stuff by heart and just wrote something like "mousePos.X" rather then exactly how the fucking API exposes it to you on a sheet of paper. As if it actually matters if something I write on a sheet of paper compiles, as if it won't be working two seconds after I step in front of an actual computer.
What kept my spirits up during my last bout of unemployment was to be combative when faced with stuff I found stupid. I just think to myself "this company actually values this person's opinions. Why on earth would I want to work here?" When they ask you for your salary requirements, and you don't actually want to work there, just double or triple your actual requirements. "What salary would I require to work here? At least 300k..." It's good fun, a lot of HR monkeys get off on thinking the people they interview would suck them off for a job, they're not necessarily ready for you to have a spine.