As much as one trip pony makes sense by itself, it doesn't make sense as it is applied to situations today. People say that something is a one trick pony when it, essentially only has one trick.
For example, I might say, "The Forty Year Old Virgin turned out to be a surprisingly good comedy, although my low expectations may have worked in its favor. I really went in thinking it would be a one trick pony."
Some that I hate:
I could care less...
It's a mute point.
supposebly
My wife often uses "if anything" in place of "if nothing else".
I've always said, "all of the sudden," and it should be, "all of a sudden."