#4 You took on a new debt
The best case scenario you could have is that you have managed to pay off any debts you may have had (be they credit card debts or mortgages) before you are set to retire. However, even if you have some leftovers, it is not going to be the end of the world. What will definitely set you up for ruin is if you happen to pick up new debt while you are already retired—in most cases, this means that you are set up for disaster, and you may end up living beyond your means.
If you plan on doing that or you think that may end up being your case, you should make sure that you take all the proactive steps to get rid of it as soon as possible. You can either opt to refinance one if you see lower interest rates are starting to be available, or you can do debt consolidation if you have multiple debts with high-interest rates going on at the same time.
If you end up with another debt soon after you retire, it is a clear sign that you are going to end up bankrupt sooner than later, and you should start making some calculations unless you want to end up having to take a part-time job for some extra income.