there will be another "mistake" in somewhat equal scope to offset each other, hence rendering the model or budget still valid.
The idea behind is that things will anyway fall the way they fall and we cannot control for that.
The optimistic view is that this theory rejects Murphy's Law, the negligent take on it is the "Trust me, it will be OK" approach to life.
Decide where you want to be.
Accountant 2: oh, fudge... product Y's revenue is inflated by $980K...oh wait... based on the Theory of Offsetting Mistakes all is well, the numbers we gave the CEO are still valid, yay.
Girl: oh, crap... your condom broke
Dude: oh, fudge... you miscounted your days, your period is due today at 8pm.... you are not pregnant, yay.