There's always a surprise, yes, but if your savings rate is high enough, and your income is decent, it's no big deal.
With your new income, these random expenses will be annoying, but you'll see your budget stabilize anyways, as your base spending of X + a Y item tends to be pretty similar month to month. :)
Some months it may be a car repair, others paying your insurance premium for the next 6-12 months, others buying a plane ticket to visit a sick relative, others a charity event that caused you to donate more than your normal amount, etc. etc.
Good to know and track these, as they'll continue to happen, even in FIRE, and you can set a budget around even these, which become known unknowns.