When I send a small transaction (< 0.5 BTC, for me at least; I don't know where the actual line is), the BitCoin client pops up a warning that says that I'm "over the size limit." As I understand it, the fee is imposed for very small transactions (which makes sense, as I've sent large transfers before and not seen it)-- so the string should probably be changed.
Edit: looking more at the code, since it can be both over and under the size limit and trigger that error, it's more complex; I'll work on a rewording and submit a pull request when I have one.
