There's nothing worse than spending two and a half hours at the pediatrician's office trying to obtain a urine sample from a two year old without catheterizing, running with her into their bathroom everytime she says "Po-po!" (all 13 times) only to have her refuse at the last second and hold it in. (Every child will eventually go, they tell me. You don't know this child I tell them).
Nothing worse except sending her home with Daddy and a collection cup and saying "Good luck!" only to learn that all dh had to do was tell her to go (as if I hadn't thought of this).
(The potty overflowed because she'd held it for more than three hours -- I told them they didn't know that child).