Why FP&A Models Don’t Break Suddenly. They Break Quietly.

How financial models slowly lose coherence and turn into manual consistency-checking exercises.

Most FP&A models don’t break because of wrong calculations.

They break quietly.

One assumption gets updated in one sheet.

Another assumption stays outdated in a different file.

Someone tweaks a number somewhere, but the rest of the model never fully reflects it.

And slowly, the Excel file stops being a single model.

It becomes a collection of sheets that are only partially connected.

The interesting part is that nothing looks obviously wrong.

The numbers still “work”.

You just stop being fully confident about what actually changes when you adjust something.

And that’s when FP&A quietly shifts:

from a decision system

to a manual consistency-checking exercise.

In practice, the hardest part is not the calculations themselves.

It’s keeping the system coherent as complexity grows.

Because once that coherence is lost, real what-if thinking becomes much harder than it should be.