A survey from Discover is showed that 2,000 financially responsible consumers age 18 to 32, basically failed when it came to their knowledge of credit cards.
The following mistakes were the biggest and most costly takeaways from the survey.
So, even if you claim to be an expert, take a look to see if you're making the same mistakes, and how your knowledge compares.
Opting into over-limit protection
One of the most unsettling findings from the Discover survey is the frequency with which millennials are racking up the debt more than their credit limit.
According to the survey, 44 percent of respondents said they've either missed a payment, paid a late fee, had to work out a payment arrangement or gone over their credit line at some point.
Since you're not able to go over your credit line without specifically opting into over-limit protection, that's a sign that Millennials aren't as savvy about their cards as they might think.
Opting in is problematic for a couple of reasons. First, when you choose to allow over-limit protection on your credit card account, you're opening the door to even more fees and interest than you're already paying.
Any time you go over, your card issuer can slap you with a fee and federal regulations don't limit how high the fee can be.
Unlike overdraft fees, the credit card company can only charge you one over-limit fee per billing cycle, but that's still money being added to your balance that you're not going to have to pay interest on.
The other problem is that maxing out your credit cards or going over the limit also puts your credit score at risk.
When lenders are checking your credit, one of the things they look at is how much debt you're carrying, versus your total credit line.
This is called your credit utilization ratio, and the higher this number is, the riskier you appear.
If you're planning to make a big purchase in the future such as a new car or a home, keeping that ratio as low as possible can make it easier to get approved, so you're not doing yourself any favors when you opt into over-limit protection.
Your credit score stays the same
One of the biggest misconceptions about credit scores can be summed in a CreditKarma commercial, which features two friends chatting about their credit scores.
Friend A makes a comment about being able to check her credit score for free, and Friend B responds by showing offer her new credit score tattoo. You know those change, don't you? Says Friend A to which Friend B responds with, Duh, tattoos don't change.
It's good for a laugh, but it's too close to the truth for comfort for many 20-somethings.
In the Discover survey, only about 1 in 4 millennials said they knew their score and less than 20 percent had checked it in the previous six months.
If you've been using credit for awhile and claiming expert status, assuming that your credit score hasn't changed since the last time you checked it is a major fail.
Credit scores are fluid, and it doesn't take much to send them spiraling downward.
Something seemingly small like paying your bill late once or twice can knock big points off your score and getting them back doesn't happen overnight.
Even if you don't generally carry a balance each month, your score can fluctuate up or down slightly as you charge new purchases and pay them off.
You have to pay for your credit score
While 99 percent of 18 to 22-year-olds in the Discover survey know what a credit score is they aren't as clued in about how to get their hands on one.
These days, there are plenty of ways to get access to your score without spending a dime.
Several credit card companies, including Discover, now offer free access to your credit score when you open a credit card account. Discover's credit score is a FICO score that's calculated based on information in your TransUnion credit report.
Citibank, on the other hand, calculates their score using your credit file from Equifax.
If you don't have a card with a bank that offers free FICO score access, you can still see yours at no cost if you use the Mint budgeting app.
Sallie Mae offers free scores to eligible student loan borrowers.
Many credit monitoring services now feature free FICO scores as well, but you'll want to read the fine print carefully to make sure there are no hidden fees.
With so many free options to choose from, there's no reason to be paying for your score if you don't have to.
One thing to keep in mind is that there's likely to be some variation in your score if you're getting it from multiple places since they're based on different models.
Pulling your credit report to check for errors or inaccuracies is a smart move if you notice that one of your scores is much lower than the others.