Three statisticians go out hunting together. After a while they spot a solitary rabbit. The first statistician takes aim and overshoots. The second aims and undershoots. The third shouts out "We got him!"
Ok this isn't a maths joke but my lecturer did it:
There were two dots on the White board all lecture (looked like this: . . ) and we all just thought it was writing that hadn't been rubbed away properly.
Then at the end of the lecture he said it was a drawing of a white cat.