The clips below demonstrate some of the most common misuse errors that we CPSTs see at seat check events.   Unfortunately, these common mistakes, which may seem rather 'small', can have deadly consequences.  The first one is a loose install. 

A properly installed carseat will have less than 1"inch of movement at the belt path.  (1 inch - that's about the size of a quarter.)  If you can wiggle your child's carseat (at the belt path - not the top of the seat!) more than that...it is not installed properly!  

In this first clip, the seat appears to have about 5-6 inches of slack - about the size of a can of soda.

The carseat in the foreground is installed correctly - with less than 1 inch of movement at the belt path.

The carseat in the background is the one with the faulty install.  Watch how much more the dummy in the rear seat is thrown about - (Take special notice of his head/neck!)

Proper harness placement is also extremely important. 

If your child is rear-facing, the harness straps need to be at or below their shoulders.  If they are forward-facing, the harness straps need to be at or above their shoulders.

This next clip shows what can happen if the harness straps are improperly placed (below their shoulders FF).  :-(

And this last clip shows what can happen in you accidentally thread the seatbelt (or LATCH belt) through the wrong belt path.   This seat has been installed forward facing, but the seatbelt has been placed through the rear-facing belt guides.   (This is a very common mistake, and one we see quite often at seat check events.)

Please follow the manufacturer's directions when installing all carseats, and if at all possible have them inspected by a certified CPST.

(Reminder: Not all police officers/fire fighters are certified, so please ask to see credentials if getting help from your local police/fire agency.)