WARNING: Avoid pulling the trigger to initiate hammer fall when the pistol is field stripped.

I got my first Beretta, and first hammer fired gun, an M9. I ended doing this the first time I field stripped it. I think I was trying to let the hammer slowly fall because I stripped it with the hammer cocked, but it ended up being a regular trigger pull hammer fall.

Why does Beretta not want us to do this? Is doing it once a big deal and is there any damage I should look for?