Such a long list to choose from....
The worst was when I painted the interior of my 57 Corvette. It was the first car I ever painted...ONLY car I've ever painted, as a matter of fact!
The exterior (black) was completely finished. I masked off the front cowl area, all around the passenger compartment and then sprayed the Venetian Red lower dash. Let everything dry, removed the masking and thought everything was A-OK.
Next time I pulled the car out into the sun, I saw about a million little Venetian Red specks on the deck lid, the quarters and the bottom halves of the doors.
That red paint went EVERYWHERE! I had to color sand and polish the doors, quarters and deck lid. Only the hood and the front fenders didn't get damaged. Luckily, I had laid on a lot of lacquer so I didn't have to re-paint the black.