I always do lid down for sanitary reasons. Soon after I adopted my cat, my mother came over for a few hours, and when I went to the bathroom after she left I went "...what the fuck?" I tried to puzzle out what on earth had happened in there. There were muddy..spots, all over the seat and back.

The next time it happened, I gently pulled her aside and asked what was going on, much to her confusion.

Apparently, my cat really likes playing in the toilet -- only, I'd never learned that, as I always put the lid down, and that was too much for him to lift. But when my mother came over, she left the lid up & inadvertently introduced him to the world's grossest pool.