[greymule]

Helmets do come off during action, and these rules cover that possibility.

"In cases where the batting helmet is accidently hit with a batted or thrown ball, the ball remains in play the same as if it has not hit the helmet."

If the runner from first is stealing and loses his helmet and a batted ball hits it as it lies in the base line, the ball is still in play. The runner is not considered to have been hit by the ball. If a thrown ball hits the helmet, it's just as if it hit the ground. Or if a helmet flies off as a runner slides into 3B and the ball hits it, the ump doesn't call interference. It's just a bad break for the defense.

"If a batted ball strikes a batting helmet or any other object foreign to the natural ground while on foul territory, it is a foul ball and the ball is dead."

Maybe on a suicide squeeze the runner's helmet flies off and is lying in foul territory when the bunted ball hits it and deflects fair. Or maybe the batter loses his helmet into foul territory as he swings and hits a high pop foul. F3 and F2 play Alphonse and Gaston, and the ball falls between them and hits the helmet. It's foul; it isn't fair if it deflects fair.

There is a minor ambiguity. The first statement does say, "The ball remains in play," but that assumes it wasn't a foul batted ball in the first place.

Incidentally, players could leave their gloves on the field up until around 1950, I think. Maybe earlier. In the very early days, they even left jackets lying in the infield. The ball was in play even if it went up a sleeve.