Max landing flap
I have a very old and admittedly vague memory of this.
A320 initial (=CFM) flight testing showed up a vibration (can't remember where) at 40 flap that wasn't there at flap 35.
That didn't bother anyone so it was certified to 35.
The IAE flight testing followed the original CFM pattern and, guess, no vibration at 40. Bingo!