Don't you hate when....

[Cap'n]

or...

you're moving rocks around in the tank and feel something squishy. "Oh, didn't realize there was a mushroom under that rock.", you think as you turn it over and see that your finger is mashed down on a giant, squirming bristleworm. Great, another swollen itchy finger.

Not as bad as a broken toe (been there) but far more frequent.