Never a good idea, especially if you're only 15 months old, and even if there are anti-slide doodads stuck to the bottom of the tub and your mom is only a couple feet away.
Just ask Stephen. This morning he busted his lip (yes, I'm from TEXAS, and I said busted...just like I said doodads previously) and he even chipped a tooth! At least he's not stuck with it; permanent teeth will be here in a few years... Until then his smile will look like mine.
This happened just before naptime, so he's sleeping now. He'll probably have a swollen mouth when he wakes up, but I think otherwise he'll be fine. Probably won't even learn a lesson from it and will be up dancing next bathtime.
Since he is OK, I'll just go ahead and satisfy your curiosity because I'm sure you're just dying to know how I chipped an incisor. It's not really that interesting of a story, I'm afraid. I just bit into a big bite at breakfast one morning and misjudged where my fork was. Ouch! I bit right into the fork and chipped my front tooth leaving my smile with a little extra bit of character.