Bills sent to residencies east of Airport Road and north of N.C. 54 failed to take into account payments customers had made since the last billing period.
OWASA realized the problem when a customer called the company regarding his statement.
"Some payments made were not posted correctly," OWASA Public Affairs spokesman Greg Feller said. "Some items were not shown as credited."
The error was caused by a glitch in the updating of the system software.
Kevin Ray, OWASA director of finance and customer service, said the company was unaware that updating the software would result in billing changes and that the checking system did not catch the error.
"Our normal procedures failed us," Ray said. "This is an application we've been using for several years."
Ray said that when OWASA officials heard of the problem, they quickly investigated but do not yet know the exact cause of the glitch.
"We've immediately taken a look at the procedure," he said. "It's difficult to say what the problem was."
Ray said OWASA is not responsible for printing the bills and that during every billing period, the printing company sends sample bills back to OWASA to be checked for error. Ray said the sample bills for this period were correct.