The STM's Opus card is once again prompting regular transit users to take an extra-careful look at their monthly bank statements.

Earlier this summer at least 100 commuters were charged twice for a pass, and while the STM promised to clear up the problem, their fix is not perfect.

Alex Harper paid twice for his July transit pass because his payment was not registered the first time.

He has been reimbursed. but for August, he had to pay twice. Again.

"I'm pretty frustrated," said Harper.

He noticed the problem right away.

"It didn't work, it did the exact same thing again. I was making a joke about it as it was happening with a couple of friends of mine, like oh watch it's going to happen again, and it really did happen again. I was furious," said Harper.

Other metro users, like John Truss, have reported similar problems.

"I put twenty dollars worth of tickets on it and went to use it the next day and they were gone," said Truss.

The STM's Marianne Rouette initially blamed Canada's debit card system.

"It seems that the Interac network asked for modification of their system, so that created like a bug," said Rouette.

But in an email to CTV News, Interac said it has made no changes to its system this summer.

Now the STM says that users may be taking their Opus card or debit card out of the payment machine too quickly.

Or a commuter's Opus card, or an individual kiosk could be broken.