It pays to keep meticulous records of your work and know how much is owed to you. On another thread, a mystery shopper was double-paid for an assignment. She identified the overpayment, contacted the MSC, and refunded it to,them. On this thread, the mystery shopping company had an oversight and missed paying the contractor for a portion of what he was owed, although they paid him when he identified the shortage.
Companies make mistakes, too. Sometimes the mistake might be to pay a contractor too much. Sometimes it might be to pay the contractor too little. Hopefully, the contractor knows how much money he should be paid and is monitoring it.