My parents used to have two cats, but only one cat basket, so if they both needed to go to the vets at the same time, one went in the cat basket, and the other went in a Heath Robinson contraption of a round, wicker shopping basket, with a string bag tied over the top to keep the cat in. As this was not a totally secure effort, mum used to have to have this basket on her lap, whilst dad drove, so she could foil any escape attempts. The cat rewarded this by peeing copiously - through the wicker and onto mum's lap. Mum was not best pleased!
I once accidentally shut one of our cats in the front room - when I realised, and let him out, there were no signs that he'd been caught short in there (and he hadn't been in there long), so I assumed all was well. Later on, I went in to watch TV whilst I had my lunch and the dses had their nap. I sat down on the arm chair - which looked fine - but soon realised I was sitting in dampness. The little furry bastard had pee'd on the chair, and the fabric cover had dried, so it looked dry, but the foam inner was soaking, and soaked back out when I sat on it. 
< boak-face
Luckily it was a cheapy cane chair with a cushion that came off, and could fit in the washing machine, so I washed it and tumble dried it - after I had changed all my clothes and had a shower. That was not the restful lunch I was planning.