My favorites are the days when you come home and you realize that something isn't quite right....you're not sure what it is yet....and then it hits.
For a long time, every night there would be a Great Pen Migration in my living room. I would wake to find every pen I owned and left out had been swatted around the floor by the cat (he says they were trying to get away and he stopped them) and left on the living room floor.
One day I came home from work and there was an enormous feeling of "not quite right". I finally got a good look and my cat formerly white cat now had a screwy, drooly orange streak down his side and my dog's white paws and ruff were orange too. Near as I could tell, the cat had found one of those smelly orange markers to swat around, dog played too, and they ended up coloring each other. Thankfully it was Crayola and came right out of fur and the carpet, but the look of Whaaaaaat??? on those two faces sent me right into giggles.