DENVER – Over 800,000 MasterCard and VISA transactions at Wal-Mart stores on March 31 were double and triple charged due to a computer error at the card processor. First Data Corporation processes card transaction for at least 500 credit unions who are members of the PSCU Financial Services Cooperative, along with other credit unions which also process their transactions with First Data. PSCU said that there had been no real fallout for its credit unions from the glitch and that, in most cases, these sorts of processing errors are caught much earlier in the process so that they never become an issue. Representatives for both First Data and Wal-Mart said that the erroneous charges have already been corrected. The charges were made to MasterCard credit card accounts and VISA debit and credit card accounts. First Data would not comment on detail about how the problem occurred but noted that the firm's quality control measures had caught the problem soon after it had begun to develop.