Set a Max Tip Amount & Add ability to remove "Custom" Button
In any situation, no one should logically intentionally tip beyond the total of a bill. The tip should be capped at 100% of the bill - if even that, to be generous.
If for some reason, there is a case where it would make sense to tip beyond 100% of the bill, there should be an option for merchants to set a cap with a verfication pop-up to the customer (and allow merchant to see as well):
I.e.
"You would like to tip $5000. Is this correct?"
Real-life example:
We have a restaurant w/ many older customers, and put in this new Clover system.
He was paying w/ a debit card, and accidentally touched the "Custom"' button, and possibly thought he was entering his PIN. He ended up tipping us over $560. His bank was charged over $600+. We voided it immediately, but it took almost all day to resolve and make sure money will not move, although it showed as missing from his account already. The customer had to verify through his bank, and we had to verify w/ our credit card processing company, and our register, to make sure the transaction was indeed voided and will not move. This wasted several hours of everybody's lives.
Unfortunately, this customer needed his funds that day and did not have the luxury to see this missing for a day. Not to mention the bad taste you would be left with after having this experiece w/ any restaurant.
Fortunately, he is a regular, and I was able to talk him through the process, and show that I indeed was going to follow through and make sure his money did not end up in our bank.
This is such an avoidable pain, that merchants should be able to opt out of experiencing. Plus, it would reduce credit card fraud with businesses having any dishonest businesses charging tons of money for TIP. There are many stories on the internet about that sort of experience as well. Keep everyone honest.