It was something in between bad mead and vinegar, tasted sweet and sower at the same time, but in an unpleasant way.
This was almost certainly an introduction of a foreign yeast, possibly airborne but probably on a utensil, in your carboy, mixing pail, etc.. If you are not absolutely scrupulous about sanitation and keeping the right level of water in your airlock, undesirable yeasts can get in there.
I'd love to hear about some of the recipes y'all have tried. I usually make about two or three batches a year and have made straight traditional mead (water, honey, yeast), orange spiced melomel (mead with fruit), white grape melomel, green apple cyser (mead with apples), agave mead, chai tea mead, and most recently apple pie mead. Of all of those, the apple pie mead was my favorite and the chai tea was my least favorite.
Have any of you ever used a dry wine or champagne yeast? I have always used one variety or another of semi-sweet wine yeasts, which produce a traditional flavor and sweetness, but am toying with the idea of fermenting a batch of my apple pie mead to finish very dry, and then mixing it with my sweeter apple pie mead to dry it just a bit more.