Concentrations of delinquent debts tend to be higher in areas that were hit hard in the financial crisis, Ratcliffe said. The average amount in collections is $5,178, and the median, $1,350. (See our slideshow for the states with the highest percentage of residents with a debt in collections.)
But for some people, the fact that a debt collector is hunting for them is a surprise. About 10 percent of consumers in the dataset had a collection out for amounts of less than $125. These were things such as unpaid parking tickets, delinquent gym membership fees and utility bills, among other more trivial debts. Some owed less than $25.
"Even if you think you're very responsible with your bills, it's not hard at all to end up with a collection on your report," said Gerri Detweiler, director of consumer education for Credit.com.
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Medical debts are a common surprise collection, with billing system problems like a bill not being submitted to the right insurance or a bill that's in dispute by you, the provider or the insurer, she said.
There are also so-called last bills. "That's when you move, and there's a last utility or phone bill, but you don't get it because it doesn't come to your new address," said Detweiler.
Expected or not, a debt in collections can make a dent in your credit score. Depending on other aspects of your score, such a black mark could knock off 50 points or more, Detweiler said. Consumers with better scores otherwise, or a more recent collection account, would see bigger damage.
Spotting a delinquent debt is easy. Federal law entitles consumers to one free credit report listing open and recent accounts, from each of the three big credit reporting bureaus every 12 months. Access them at AnnualCreditReport.com.
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Handling a debt in collections can be a little tougher. If it's an outright mistake to the tune of, "I paid that bill ages ago!" you'll need to dispute the error with the credit bureau. Paying the debt doesn't necessarily remove a collection from your report, and such black marks can stay on your report for seven years, said Ratcliffe. Some credit score calculations don't weigh a paid collection account any differently than one that's outstanding, either.
"The time to negotiate is before you've paid it," Detweiler said. Ask if the collection agency will arrange to remove the debt once paid—many will, particularly if it's an unexpected debt you weren't aware of.
—By CNBC's Kelli B. Grant