I didn't realize this, but gas stations can put an "authorization hold" on your debit card for an amount separate from your actual charges.
The wikipedia entry
http://en.wikipedia.org/wiki/Authorization_hold lists gas stations, restaurants, motels and rental car companies as the most frequent users of this practice.
Anyway - I wanted to share my experience this week, to give you all a heads up on this practice.
I went to the gas station on Monday. The charges were $28.14. I went inside to pay with my debit card. It was a new clerk who was having trouble with the credit card machine. She asked for my card twice and had to call the manager out to finally get the transaction to go through. Today, I look at my account online, thinking I have about $300 as a balance, it was $50. There was a $250 pending transaction from the same gas station and a pending transaction for the $28.14 actual charge. I went to the gas station, they checked their end of the day print out from Monday and only showed the $28.14 as being submitted for payment. They did say that the $250 charge has happened before and that it should be credited back within 72 hours.
I called the bank. They said it is called an authorization hold, and if I had other transactions that go above the current $50 balance, I will incur overdraft charges. Luckily, I don't have any outstanding checks, but, I also couldn't get groceries today after work.
I was not warned that there would be a $250 hold on my account and I didn't authorize it. The entire thing is a small inconvenience, but in another situation it could have cost me over draft charges, or at the wrong time kept me from having needed cash that is in my bank account, but put on hold by the gas station.
The bank said that they have seen this before, but usually it is only for $1. The wikipedia entry also mentions it is a common practice at $1 for gas stations to do this when you pay at the pump. The bank said that the $250 hold was legal and the practice has been used for years.
So, after looking into it a little bit, my advice is: Don't use your debit card for hotel/motel rooms or rental cars (unless you are very comfortable and carry a large balance) and also beware that restaurants can put an additional hold amount on your card and I would pay inside at the gas station. (Even tho I paid inside, I think the teller may have screwed something up as the info I read and from the bank indicates that this practice is almost exclusively used at the pump when you swipe your card before pumping the gas - not at the counter when the teller has the correct amount)