Dollars Equal Cents
Theorem: 1$ = 1c.
And another that gives you a sense of money disappearing.
1$ = 100c
Here $ means dollars and c means cents. This one is scary in that I have seen PhD's in math who were unable to see what was wrong with this one. Actually I am crossposting this to sci.physics because I think that the latter makes a very nice introduction to the importance of keeping track of your dimensions.