What you're dealing with is a ghost sale. It can be quite common on high volume campaigns. Ghosting happens when a user clicks through your ad and gets recorded in your pixel tracking system and receives your cookie, but then clicks through the ad of a competitor on the same network and makes the purchase from them.
On most networks, when a user hits the thank you page, every affiliate that has a pixel placed will receive an initial pixel fire. Your pixel tracking software then checks to see if that user has your cookie and records a sale if they do.
Because the user does have your cookie it gets recorded as a sale. Its not an error, its a coincidence.
