It more negatively affected the customer I was serving, but when I worked in food services, for all of two weeks, the credit card processing machine was sticky from someone spilling soda on it and the buttons stuck and gave an extra press or two.

So I charged a couple over $2,000.00 for two sandwiches, fries and medium drinks when trying to quickly enter in the charge and hit enter.

The manager spent ~20 minutes on a call with the payment processor and was able to reversed it and all was resolved but I panicked there for a bit.