It happened to me.
Over a decade ago, when I was working in another industry, it happened to me. In a crowded room, an older man came up behind me, said hello, and as I turned around in those close quarters, he snatched my hand and placed it on his crotch. It wasn't the first time something like that had happened to me (I'd had my behind pinched more than once), and it wouldn't be the last, but at that moment I froze. I didn't tell anyone at work about it, because there was no evidence it happened, and I was afraid of being called a liar and a troublemaker who was just trying to get attention.