WHY WOULD I WANT $400 WORTH OF GAS IN A SINGLE CAR?

My credit card would not work at a local gas pump Wednesday night.  So, I walked into the store to see if they could swipe it there.  I told the clerk / attendant I wanted $40 on the pump.  The gas guy keys in $400 and I told him I didn't ask for $400.  The guy looking dumbfounded said, "you didn't?" - Stop.  Why would I want to buy $400 worth of gas for a sedan - a mid-size car and where would I put it you nincompoop? He credited it back to my card...I'll definitely have to call my credit card company to follow up to ensure I'm not charged...Jeez!