I travel about 5-6 months out of the year. Just about every month I have 1-2 add ons to my credit cart. It is usually by changing a tip number from a 4.00 to a 6.00 or something like that. They change them and bet that the person is not going to call since it is only 2 bucks, and when you do call they say it was a mistake, or they couldnt ready your hand writing. One manager told me it happens quite a bit and they have fired people for it. So it may be a pre charge thing but it happens where a wait staff member will bump the tip up. Just check against your statement and call the store.
