I made my first buttercream from scratch for my Birthday Surprise Minnie Mouse cake. The recipe was from Elisa Strauss's Confetti Cakes book and seemed simple enough (she called it Italian buttercream, but I later found that it was actually a Swiss buttercream).
Anyways... after adding the butter and beating for what seemed to be forever, it separated and looked horrible. I thought I'd done something terribly wrong. So I tossed the batch and went to the store for more supplies. I came home and checked out YouTube to see if there were any better instructions. It ends up, I just had to beat it another minute or two, and it would come back together.
Sure enough, the 2nd batch looked terrible for about a minute, and another minute later I had an awesome fluffy buttercream.

Lesson learned.

(BTW-- the cake turned out awesome. I was really happy with it, and so was my friend's 6 year old daughter Kate.)

-Alex