This is why it's so important to set up automatic payments to ensure your bills get paid on time, all the time. With credit cards, you can set up automatic payments that take the minimum payment out of your checking account to ward against a late payment. You can always make a second payment that reduces your debt or pays it off entirely. You can sign up for automatic payments on the website of your card issuer.

Settling a credit card debt

All the advertisements about "settling your debt for pennies on the dollar" make debt settlement sound like a great solution. But failing to pay what you owe a creditor will take a serious toll on your score.

The 680 scorer would lose 45 to 65 points with this maneuver, while the 780 scorer would shed 105 to 125 points.

Our scenario assumed that our borrowers would miss one payment before settling the debt with their credit card companies. In reality, debt settlement negotiations can drag on much longer, with each missed payment taking another chunk out of your score.

Settling a debt with a collection agency would hurt less, probably much less, because the FICO formula is set up to weigh more heavily what the original creditor says about you than what a collection agency reports. But if our borrowers were settling with a collection agency instead, their scores would be lower to begin with, because they would have collection accounts on their records.

Also, you should know that the amount of debt your creditor "forgives" in a debt settlement solution is typically added to your taxable income. So you may save some money by settling a debt, but you'll give some of it back to Uncle Sam in higher taxes.

Losing a property to foreclosure

Foreclosure deals a severe blow to your credit score: 85 to 105 points for our person with the 680 score and 140 to 160 points for the one with the 780 score.

Foreclosures have implications for your future ability to get a mortgage as well. Although your score may start to improve as soon as the house is gone, mortgage lenders may not be willing to extend you another home loan until two to four years have elapsed.

In an attempt to protect their credit, many people attempt short sales, selling their houses for less than what's owed, with the lenders' permission. Unfortunately, these transactions, even if successful, are often reported as settlements. And a settlement, as you've seen, is pretty bad for credit scores. To lenders, a short sale isn't quite as bad as a foreclosure, though, and it may be easier to get another mortgage once you've rebuilt your credit.

Filing for bankruptcy

FICO spokesman Craig Watts once called bankruptcy the nuclear bomb of credit actions. Filing for bankruptcy would shave 130 to 150 points from the 680 score and 220 to 240 points from the 780 score.

This is different from the other black marks, where the higher scorer was still left with better numbers than the lower scorer. In this case, both would wind up near the bottom of the credit barrel. Getting new credit, particularly in the current credit-crunch environment, would be extremely tough.

Sometimes, of course, bankruptcy is the best of bad options. But if you can't pay your bills, you should at least explore the other possibilities: forbearance, credit counseling or even debt settlement. Finally, if you have any of these five black marks on your record, remember two things: The impact on your score may differ from what's shown above, and regardless of how many points you lost, you can rebuild your FICO score over time.

You can start by using a free FICO score estimator, such as this one at Bankrate.com, or MSN Money's credit score estimator, which similarly models a score on Experian's 330-to-830 range, to see where you stand.

Or you can sign up for free credit scores from sites such as Quizzle, Credit.com and Credit Karma, which use the actual information on file about you with the credit bureaus. But the scores you get still may not be the ones lenders actually see.

Or you can buy your Equifax or TransUnion FICO score from myFICO.com. (Experian no longer sells FICO scores to consumers, although it continues to sell the scores to lenders.) With paid scores, you'll get specific advice about how to improve your numbers. In general, when you're trying to build a credit score, you should:

• Pay your bills on time, all the time.
• Reduce your credit utilization; below 30% is good, below 10% is better.
• Have a mix of credit on your reports, including installment loans (mortgages, auto loans and personal loans) and revolving accounts (credit cards and lines of credit).
• Refrain from closing accounts.
• Apply for new credit sparingly.

Liz Weston is the Web's most-read personal-finance writer. She is the author of several books, most recently "The 10 Commandments of Money: Survive and Thrive in the New Economy." Weston's award-winning columns appear every Monday and Thursday, exclusively on MSN Money. Click here to find Weston's most recent articles and blog posts.