I did not know this:

A co-worker used her credit/debit card to purchase gas at the pump (like most of us do).  She received her receipt like normal. However, when she checked her statement, there were 2 $50 charges added in addition to her purchase. Upon investigation, she found out that because she did not press the 'clear' button on the pump, the employee inside the store was able to use her card to purchase his/her own gas!  To keep this from happening, after you get your receipt, you must press the 'CLEAR' button or your information will be stored until the next customer inserts their card. Be sure to tell all your friends/family so that this doesn't happen to them!