There was a story in one of them climbing mags a while back, by "Big Wall Pete" Takeda if DFA is not mistaken, outlining the author's first foray into the aid realm. He didn't have a helmet, but didn't really figure he needed one (he may have been roped-soloing or something; in any case, he was on the sharp end and reasoned nothing was going to get knocked off on his head), and off he went. After something like four or five hook placements popped and cracked him square on the noggin, he retreated to the Valley floor, blood pouring down his face, to rustle up a helmet.
Quelle grande amusement!