I just encountered this one:
How many holes are there in this shirt? Picture
There are two tricks in the answer.
The official answer:
Eight. There are the two obvious holes torn in the front of the shirt. Plus there are the four normal holes for your head, waist, and arms. And you can see the background color through the two torn holes, so, according to the official answer, there must be two more holes in the back of the shirt.
We can’t see the back of the shirt. Therefore we have no way of knowing if we’re seeing the background color through two small holes or one large hole in the back. The answer, based on what we can see, could be seven or eight.