A great way to fuck with register-jockies is to hand them a $20 bill to pay for lunch, then AFTER they ring it into the God-like machine that calculates change for them, throw a different combination of cash at them and watch the fireworks. A manager is definitely getting involved in this complex transaction, as either a manual (good God!!) calculation needs to be done, or someone has to locate the VOID button on the register and start again.