It's an all to common practice for gas stations to allow carriers to dump toxic chemicals into gas storage tanks for a little under the table cash. It happened to my father. He bought gas to put in his mower. When he started it he said it barley ran and made the octane rattle sound really bad. He drained it back into the gas can, got a different can, went to a different gas station and everything was fine. He said he noticed the next day the can with the contaminated gas had formed a skin over the pour spout, like what forms on paint when you leave the lid off to long. Another possibility is someone put something like E10 or higher alcohol doped gasoline. Here is some info on E10. They have up to E25
List: E10 Engine Damage & Performance Issues.