DALLAS (AP) — Southwest Airlines says it has begun filing refunds to customers who were accidentally billed multiple times for a single flight, after a half-price online ticket promotion backfired.

The special sale designed to celebrate Southwest's reaching the three million mark in Facebook fans was supposed to give customers who booked flights on Friday a 50-percent discount on certain fares.

Instead, hundreds of frustrated would-be fliers wound up posting stories on social media about how their credit and debit cards were being repeatedly charged, in some cases 20 or more times.

The company says it has identified all customers impacted by the problem and has begun filing refunds to the affected bank accounts. Southwest says it will also pay customers for any overdraft fees caused by the additional charges.