Not quite as disrespectful to your monetary system as using it as toilet paper, the Money Erasers will let you whip out 20 and 50 pounds and fix mistakes. Money is normally good that way, although this one only works well for pencil mistakes. If you made that mistake in pen you are just SOL. They would probably make a US version but I think they are afraid that the eraser would be worth more than the dollar.
Product Page (£3.00, about $4.40)
