An old one from about 1970. Well no in fact - I've found it is from 'A Mathematicians Miscellany' by Littlewood which was published in 1953. Oh well I would have liked to have made it up. ;-)
Teacher: Now suppose the number of sheep is x....
Student: yes I see that okay, but what happens when the number of sheep is not x?
About 2010 I think
Maniac drivers going at speed are accidents waiting to happen!
The less time I spend on the road the less chance I have of meeting one of them.
So I always drive as fast as possible whenever I have to get anywhere,