The Mucho Nacho jalapenos I grew one season had a sour taste to 'em. Dunno if the plant was really Mucho Nacho or not, but that's what it was labeled as when I bought it (it wouldn't be the first time I've bought a mislabeled plant from the same store). They did have a little bit of heat to 'em IIRC, but not much. As I've never had anything give off a similar bad taste, I'm convinced it was just the flavour of the variety I was growing and not so much caused by anything in particular.
Maybe you just ended up with a bung variety like me?