1. Using your car seat wrong! Nationally, the misuse rate for child safety seats is over 80% and as high as 95% in some areas. That scares me more then i can put into words. Please make sure your using your car seat correctly and have the installation checked by a professional.
2. Using Both the LATCH system and a seat belt to install your car seat. I understand the thinking that using both is safer when in fact its not. Use one or the other but never both. Just use the one that gets the best fit and you are most comfortable using. If you prefer to use LATCH and want the seat in the center, make sure your car has center LATCH (many do not)
3. Infant Carries should NOT be used on top of shopping carts! I know you have seen other moms do this but they are not made to work with a shopping cart. Visit here for more info
4. Forward facing to soon. even tho the law states that you can turn an infant at 1 year and 20lbs the American Academy of Pediatrics now advises parents to keep toddlers in rear-facing car seats until age two, or until they exceed the height or weight limit for the car seat.
5. Winter Coats and Car seats don't mix.
6. Chest clip belongs on the chest not the belly. Chest clip should be even with the armpits.
7. Not reading the owner manual. Always read the owners manuals for your vehicle and carseat thoroughly. They often contain specific information about carseat installation that may not be obvious. Some models may vary from what you would expect.
8. Make sure the harness is snug. You should not be able to pinch any strap or put more then one finger between your child's collarbone and the strap. The straps should be as Snug as a hug.
9. Don't buy used car seats. You never know the history of a seat when you buy used and saving money isn't worth risking your child's life.
10. Make sure the seat is adjusted correctly for your child and the way they are facing. Keep the harness straps at or below the shoulders while rear-facing and at or above the shoulders while front-facing.