I'm not FIRE yet, so I'm sure others will have more detailed answers, but here is how I'm thinking about it.
I incorporate all expenses into some sort of budget line, and we have been tracking our finances this way for several years. So we have a good idea for the "car replacement" type situations. For health care I am concerned about increasing out of pocket costs, but figure I will have a built in cushion. For example, if I decide I need $40K per year to live, I will save more than exactly $1MM. I'm not sure what that factor will be yet, but I'll definitely go over the 25x (which of course means that my SWR will be less than 4%.)
Plus we will have social security, plus I'm likely to inherit something at some point (not something I count on, but I know the likelihood in my case and it's pretty good). Plus I may consult a little or do something that earns money at some point after RE. Plus there will be some expenditures I could cut if needed to save up for a big expense.
So that's how I account for the "what ifs"--multiple layers of safety margin.