A couple of bonehead things...
I was playing at Ballybunion New about 10 yrs ago, and I left my bag on the forward tee, then went back to hit off of the back tee. I was hitting 3-wood, and even though the bag was directly in my line, I figured it would not be an issue.
Of course, I catch the ball thin, and hit it straight into my bag, or, more precisely, into the clubheads sticking out of the top. Incredibly, the ball hits my graphite-shafted driver right below the clubhead and snapped the clubhead clean off!

To add insult, that is a two-stroke penalty.
Another time, playing in NY, my playing partner forgot to put the parking brake on the golf cart. Preparing to putt, we turned around and watched as the cart started rolling down the hill toward a pond. There was nothing we could do but watch as it rolled into the water and settled in 3 or 4 feet deep.