If you are "booster training" and your kids switch between harnessed and booster seats, and are heavy enough that the harnessed seats have to be installed with the seat belt and not LATCH:
CHECK THAT THE BELT IS BUCKLED IN YOUR HARNESSED SEATS EACH TIME YOUR KID GETS IN!!!
Mine got confused and UNBUCKLED their harnessed seats without me knowing (because they can unbuckle their boosters themselves, which are in another car). They of course didn't get the potential consequences of this plus had forgotten all about it the next time they climbed into their seats. I have NO IDEA how long I was driving around with them in harnessed seats that WERE NOT INSTALLED AT ALL. YIKES! And, I could see it being easy to potentially accidentally unbuckle the seat, either by stepping on the buckle somehow or pressing on it - we all know how much crap our kids like to take into the car with them that cover up those buckles.
Check, check, check!!! Now my kids know and are appropriately freaked out. But always check!!!
I probably should've posted this in the lounge...mods, feel free to move!