This was scary.
I routinely pull a child or two in an older Burley trailer. A couple nights ago, I went to the grocery for milk, with my 4-year-old in the trailer.
I turned into the parking lot, off the street, and CRASH...When I looked back, the trailer was lying on it's side with the 4-year-old half in and half lying on the pavement.
After comforting him, looking to make sure he's not much hurt, cleaning him up (thank you, helpful people at the Lombardy St. Kroger), etc, I'm still trying to figure out what happened--I've run errands with this trailer dozens of times and it's very stable.
So we're in the store, and he tells me (this is 10-15 minutes later), "Papa, the light on the trailer isn't working--I was trying to turn it on and it wouldn't come on." At which point I realize what happened; he had turned around, kneeled on the trailer seat, and was leaning against the side--the side that was on the outside of the turn and bump--and reaching over the seatback to mess with the light.
So reinforced trailer rule; you must stay seated. And a good reinforcement of the "wear helmets in the trailer" rule--he had a couple scrapes and a split lip, but no real injuries.