Examples:
College tuition
Wedding Costs for children
Gifts
New (to you) vehicles
Property maintenance
Property maintenance can be distilled down to a monthly average. Be sure to be setting this aside in FIRE into a maintenance fund. An example might be that a roof lasts an average of XX years and mine has been on for some fraction of that, so it has ZZ years left in it. Rate of inflation is X%. Cost to replace roof today would be $X,000, therefore in YY years the cost would be about $Y,000 (apply inflation factor to X to get Y). Lay out a 10, 20 and 30 year maintenance plan with big items that you can reasonably foresee needing major maintenance. Set aside enough in your monthly budget for this, and other typical maintenance costs (pumping septic tanks, painting exterior, replacing worn out major appliances like fridge, washer, dryer, etc and don't forget an "oh shit" factor for things that just break - like my stupid kitchen faucet just did that was only 13 years old).
Ditto wedding costs - you know when the little ones will hit 18, decide NOW what you want to pay / chip in, then set aside enough on a monthly basis to be there by the time they hit 18. After that, let it ride until the joyful date arrives.
Gifts? To whom? Never mind, that doesn't matter - again, a monthly budget number. Pick your poison. Say $100 / month? Multiply by 300 or 400 and that's the money you'll need to add to your FI / ER savings to cover the monthly gift budget forever. Just stack up another 30 or 40k in your index fund and you have it covered, see?
Vehicles - same as anything else. How many years does your current vehicle have left in it given it's mileage, maintenance history, typical life for that vehicle type, how hard you drive it, etc? Make sure you're running a monthly surplus high enough to allow you to save to cover a new to you vehicle when the time comes to replace your current one.
College tuition: Set a budget and tell the little one this is what you have to offer them, if they want a more expensive choice, it's up to them to pay the difference. Save up that amount such that when you spend it, you still have enough to cover your other needs, including the above.
There's nothing magical about it - My ER / FI / IW target income takes these things into account. Initially I'll be spending a few grand a year less than my target income, so I have a surplus in ER / FI for just such "lump sum" type items. It's the difference between looking at things on a strictly cash in / cash out basis and going to a more accrual way of looking at things and planning for those known future expenses. I plan for the unknown future by having further margin.
A cash way of looking at a roof: There is a big lump expense one year in every 20 to 30 years and all other years, my roof is "free".
An accrual way of looking at a roof: I use up about 3 to 5% of the future cost of a roof each and every year, so need to be setting that amount aside each and every year such that when I get to the point where the roof is fully worn out and in need of replacement, the money is at hand to accomplish the job. (OK, not THIS simple, but it gets the idea across in a short forum post).