For me it was when a mystery account showed up on my credit report and now I had to fight to get it removed. Turned out years ago I was on a short term lease with someone who had stained the carpet in their bedroom. The management company had to replace it and billed it back to us, the tenants. I was unaware of this until seeing it show up on my credit report. After tracking down the collection agency assigned to the debt I began to get the equivalent of Navy Seal trained debt collection efforts in the mount of over $2,000. I wasn't personally responsible for the carpet, but I was on the lease after all. I did what anyone would do and asked them what the minimum amount was that they would accept to clear the debt (since of course they purchased the debt for pennies on the dollar). The process can be pretty daunting if it is considered a legitimate debt, but there are some things you can do to protect yourself when negotiating a settlement. One thing is to use Google Voice to record any conversations with the debt collectors to get them to keep their promises. These guys are shady and they are trained to be that way. For example they will always lie to you and tell you 1) they need an entire lump sum, and cannot accept payments (generally false) 2) they need it within 3 days or some ridiculous timeframe, etc. etc. If they hear 'voice recording is now on', they are much less likely to try and BS you.
It's funny how people change when they are on camera, or being recorded unexpectedly. Doing this is simple, log into Google Voice, press the call button, type in the number to the slimy bill collector, and it will ring your phone. Once they are on the call press the number 4 and you will hear 'call recording on'. Your call will be recorded and will be accessible from your Google voice account. You can even save the file for future reference.
One other good practice is to call them from google voice and record the call, then request they update their records with your Google voice phone number so you can record calls and you no longer authorize them to call your cell phone directly. Let them know it will ring the same phone. They are much less likely to refuse when they know they are being recorded.