I don't understand this saying.

Often doing the stupid thing is the right thing to do, instead of thinking hard to do the smart thing that ultimately wont be needed and will be harder to understand.

So juniors thinks twice before doing stupid simple stuff. Intermediates thinks twice and does smart stuff. Seniors only does smart stuff where it is needed and does the stupid simple stuff without thinking in most places.

well actually I am not familiar with the phrase before, so maybe KineticLensman knows the proper meaning.

That said for that specific meaning I would prefer something like - The novice fears simplicity.

Maybe throw in a - The expert loves it - at the end.

when the expert thinks twice they don't do something stupid?

That's not how I interpreted it at all.

"It's common to become very good at doing it badly"

There is another important difference: the expert thinks twice before doing anything at all.