Through testing the multi-tasking, I have reliably reproduced this glitch while listening to the radio or chatting with other sims. When the painting progress gets to about 11:00, the sim will glitch - she'll teleport a foot or two and the sketchpad will be on the floor in front of her. There is no painting in her inventory and when you click on the sketchpad, the only menu choice you have is "Paint from Reference" and the "Place in World / Inventory"
I've tried it with the sketchpad in and out of the inventory, multitasking by listening to music and by chatting with other sims, and in almost all cases it will glitch. It doesn't break reliably with chatting, if the sim you're chatting with decides to walk away before she hits the 11:00 point, it generally completes. I'll play with some other options to see if this is the case with listening to the radio, too. It definitely breaks just the sketchpad you're using. If you buy a new one, it will work until you multitask with that one, too.
I updated the original post to reflect the findings. @crinrict - I sent a link to a save file where you can easily queue up the tasks and break the sketchpad.
