The first time a waiter returns with the wrong change or a wrong calculation, it is 30% chance that the cashier is careless.
When you visit the SAME restaurant the second time and you find wrong calculation , then you can say 100% that the cashier and the waiter are cheating.
After alerting the manager, and in your 3rd visit, the wrong calculation comes up again, you can be sure 100% the the floor manager, the cashier and the waiters are in a gang cheating the restuarant owner / shareholder.
There is no excuse for frequent errors for professionals.
The same applies to fast food shop, supermarkets, convenience stores operation staff...etc
One dirty old trick is that the cashier and the waiter
send you a bill that shows a slightly higher $ amount.
Most likely victims are large tables with many people because too many items (e.g. dimsums) ordered can explain mistakes due to confusion. Besides, the one who pays for the happy party normally does not check the bill to spoil the mood.
It can be a settled or unsettled bill of other tables next to yours with similar ordered items (e.g. set meals ...etc) BUT showing a slightly higher $ amount.
If you find out the truth, the waiter plays ignorant and blame the busy cashier for the careless mistake. If you can't find out the trick at a glance and pay accordingly, they pocket the difference before settling the amount with the restaurant's accounting department.