I've fallen off 5 times in my riding career, but only one of those times was because of a buck. It was my first ride on Scout, and I insisted on riding in my English saddle. Yeah, idiot move, riding a new horse of unknown training level in the least secure saddle I own. He'd apparently never cantered with a girth tightened before. He got a little excited in the trot and goosed up into a canter, and the girth caught him and he threw a little buck that I wasn't expecting, and I ate arena dirt.
Twice, on two different horses (a year or so apart). Both during expensive clinics with a professional audience. Great timing I have, eh? For my credit, they were both spook bucks and took me by surprise to say the least.
Oh god, Chinga use to be terrible! Every ride we'd have a buck, or four! Although suprizingly he's only got me off twice and Corby's gotten me off once, although hers was a full rear, buck, wee. Type thing ... mares.