When he was a kid he had a guinea pig named Tweety, which he'd let out of the cage to run around the house. One day he was watching TV and eating nacho cheese Doritos out of the bag, not really paying attention to what Tweety was doing, or looking at the bag as he was reaching in for handfuls of chips. One particular bite of Doritos was bitter and extra-tangy and nasty (I think they're kind of nasty to start with but maybe that's just me), so he looked in the bag, and Tweety had pooped in it! My husband ate guinea pig poop!
One more amusing thing, we used to have a cat named Noodle who was a Maine Coon. He loved water. One day I turned the bathroom sink on just enough so it dripped continuously, and I put him on the edge of the sink to see what he'd do. He loved batting at the water and sucking it out of his paw fur so much that it became an obsession. You'd go in the bathroom and he'd run to follow you in, then climb up on the sink and meow. Often he'd just lay in the basin, snoozing. When it came time to brush your teeth you had to shut him out of there or he'd get covered in toothpaste froth because he just couldn't stay out of the way.
Who's got a funny pet story? Bonus points if it involves doody in some manner.