Regarding paying with $20 worth of nickels. Technically you broke the law, and the clerk could have called the police on you.
Limitation
(2) A payment in coins referred to in subsection (1) is a legal tender for no more than the following amounts for the following denominations of coins:
(d) five dollars if the denomination is five cents;
(For some reason I had to click on that link twice to get it to load.)
And to the person that pre-auth'd a card to its max; we had a person do that at a hotel I worked at long ago- to get back at a customer who was being a jerk. The customer called Visa to complain, and Visa called the hotel and told us that person was no longer allowed to process Visa payments, and if Visa found out she still was the hotel's Visa account would be suspended. So she ended up being let go.