You see Miles on the toilet at the beginning of Sideways.
I heard a story about the filming of that, but it might be hard to verify, so believe it or not, here goes…
Bathrooms and fixtures on movie sets aren’t usually hooked up to anything if they don’t have to be, and Jane’s toilet was just set on the floor. She said she naturally let go when sitting down, forgetting that it was only a prop, not real plumbing. A prop man cleaned it up and they did the take again. Fonda said this time she faked the peeing.
This? Or am I just completely forgetting a scene?
Now you’ve got me thinking I’m misremembering.
As penance I’ll offer up another: 40 Year Old Virgin.
No problem. Actually, that scene’s a Bodily Function two fer.
GARDNER: “So what’ll it be for breakfast, barbeque or chilli?”
[PHIL vomits]
GARDNER: “OK, chilli it is”
If nobody’s mentioned it yet, there’s Tom Hanks’ long urination scene in a League of Their Own. and Leslie Neilsen’s (with other noises) in The Naked Gun.
And Austin Powers’s in (I think) the first AP movie, when he got unfrozen.
In Last House on the Left, the rapists force a girl to urinate in her pants.
On that note, Star Trek V would have been the first time we saw a toilet on Star Trek… had the movie actually existed. 
It was used only as a place to sit in the brig, but Shatner, the director of that movie, says it was intended to be a toilet.