vocabulary help

So, yeah, I have a slang dictionary (and an internet connection), but this seemed like more fun.

I just ran across the phrase "banana skin". And, more specifically, the sentence, "So much so, in fact, that even two of the greatest choreographers ever - first Balanchine, then MacMillan - had a go, and found it something of a banana skin." And I /really/ want to know what that means (and, mostly, so I can start using it all the time).



Any clues?