Here's something I did in the mid 80's...
I think the law on dollar bills, is that as long as 1/3 of the bill is intact, the store must take it. So I ripped about 1/4 of a $20 bill off the left side, kept it, and spent the remaining 3/4 of the $20. Then I got another $20 bill, ripped 1/4 from the right side, and spent that 3/4 of that $20. Now I had both side pieces of a $20 bill, including the "20" from the corners.
I took a $1.00 bill, carefully cut each of the sides off so the number 1's were missing, and then scotch taped the $20 sides onto both ends. I now had a $20 bill, made from a $1 bill. Sure, it had a picture of George Washington on it and it said One Dollar along the bottom, but it sure looked like a $20 if you didn't look too closely. I went to a store, bought a pack of Life Savers for 59 cents, and collected my $19.41 cents in change. Profit!
I only did this a couple more times, finally deeming it way too risky. I wouldn't suggest anyone trying this. Seems like it's be really easy for a cashier to notice something like that. Especially since these days, a lot of cashiers will take the time to mark your bill with a counterfeit-detecting pen. It would still mark correctly, but that'd be an extra couple seconds for her to look at your fake $20.