CALGARY - A Boxing Day mix-up that saw diesel fuel pumped from a carrier truck into a gasoline reservoir at a Calgary Co-op station spiked fill-ups for about 500 unwitting customers.

A tip-off from an impacted patron led to the station attendants shutting off the tainted supply 26 hours later, but the damage had been done, said Co-op spokeswoman Karen Allan.

"About 500 customers received fuel with a mixture of gasoline and diesel but we don't know what the exact proportion of that mix is," said Allan.

"Some customers didn't have any issues but others did experience problems right away."

Co-op was able to contact the vast majority of those purchasers through its membership records, offering to pay any repair bills and also courtesy or rental vehicles, said Allan.

"We told them what to look out for ... trouble starting, lack of power, blue or black exhaust," she said.