I applied for the wrong job for my placement year. Put down COMPSCI on the form (which, it turns out, is computational biology, which I knew nothing about) rather than ITSEC, which was the software dev side of things.

I only found out in the interview, when the first question was asked:

"So Almond, I'm a bit confused as to why you've applied to this role specifically given you've no biology background at all - could you fill us in?"

...errr...

I spewed some kind of crap on the spot about wanting to work in a field where I saw a direct & differing application of computing than I'd seen before, and thought my focus on the technical, rather than the scientific side of things might be an asset to them. This awkward exchange went on for a while - but somehow it seemed to work, because I was offered the job, and decided to take it - had a fantastic year there.