SAN DIEGO -- 10News I-Team reporter, Mitch Blacher, asks the San Diego Water Department why it's making customers pay for *its* mistakes.

Some San Diegans are getting a terrible surprise in their mailboxes. It's a bill most people expect every month, but at a price some just can't afford.

"We got a back bill for over $6,000," said homeowner Mike Johnson.

Johnson is talking about his water bill, more than $6,000 in one month and he's not alone. The I-Team uncovered one bill for more than $16,000 and another for more than $21,000.

"What was your reaction to get a bill for that kind of money?" asked 10News I-Team reporter, Mitch Blacher.

"Shock," said Johnson.

But customers like Johnson didn't leave a faucet running.

The Water Department just wasn't charging them properly, then sent many of them a letter saying, `in order to correct our error, we need your money.’

"Somebody has to be accountable. It can't just be the individual victims each time," said Sue Ann Macomber who works for UCAN, a consumer advocacy group.

"I have a folder like this of complaints," Macomber told 10News.

UCAN is known as the `bulldog for the underdog,’ and has been keeping tabs on the Water Department and its back billing.

"If there's an error and whether we're alerted to it or not, with the Water Department, we pay," Macomber said.

"Is it fair for customers to pay for your mistake?" asked 10News I-Team reporter, Mitch Blacher.

"Again, I think the answer is yes," said the Deputy Director for Public Works, Mike Bresnahan, the guy sending those pricey water bills.

Bresnahan says he `has to.’ And, he blames a 10News investigation that aired a decade ago.

That story exposed unpaid water bills costing the city millions.

It showed big corporations getting let off the hook, while residents had to pay or be cut off.

The Water Department said the story forced the city to extend the time allowed for back billing infinitely.

But, ten years later, they're still finding mistakes.

So, if a mistake occurs today, you could be paying for it twenty years from now.

"At some level, the customer had to have known that they were receiving a service and not being billed for it."

"Had we thought there was something going on, had we had a clue, we could have compared our bills to our neighbors and figured out there indeed was something amiss," said Johnson.

Now he sees an extra $171 show up on his bill month after month after month.

In fact, for the next five years, Johnson will have a reminder of the Water Department’s mistake on every single water bill.

"We don't mind paying our bills. We just wish we would have known we owed this money,” said Johnson.

"We clearly regret the situation. In an ideal world, this would not happen."

The Water Department tells 10News only a small percentage of ratepayers have billing problems and its now using a computer system to reduce the number of errors.

The customers 10News talked to for this story say the Water Department has been willing to work with them on payment plans.

The Water Department also says it never intentionally back billed, but it's legally obligated to collect.