3 for me, two on the same locomotive. 1st was when my son was 3-4ish. I'd run the train on the floor, and he'd run after it. Once he tripped and fell on the train. After I knew he was okay, I checked the damage. Broken connecting rod on one side of the steamer, and popped both trucks off of the tender. Got new rods ordered, but the tender I had to glue back together as MTH didn't have replacements anymore.
A couple of years later, I made a small layout that rolls under a twin sized bed. The floor is tile in the bedroom. I had my nephew in my lap driving the same train, when just as it got to the O36 curve he opened the throttle all the way. Those PS-1 MTH locomotives take off. Before I could slow it down, it left the track and hit the floor, sliding along the tile. A dent on the boiler and bent handrail is all, and then you don't see it unless you look for it.
A couple of months ago, I got a new K-Line GG1 with TMCC. I bought a used base and pair of Cab-1s to try command out with. Well, ended up the base was off frequency, and the handheld wasn't completely together so light leaked into teh speed control. The GG1 took of when I tried to slow it and it came off the curve and slid along the bare concrete floor. I was luck in that the ladders stick off of the body, so the black got scrapped off of them, but the body was fine. I still need to touch that one up.
So new rules at my house, kids only can run trains on the carpet layout, no running is allowed, and new to me equipment gets tested on same carpet layout.