A new study by Dr. Shital Parikh indicates that car seats should be used carefully outside of cars. Babies can fidget and tip themselves over when placed on improper surfaces. And, if the restraints aren't used, babies can fall from the carrier while being carried or placed on a raised surface. Between 2003-2007 over 40,000 babies were sent to the emergency room after falling from (in their car seats) counters, tables, sofas or beds. Many serious fractures were reported and 3 infants died. In addition to injuries from falls, babies can suffocate if they fall on a soft surface or strangle on the restraints.
Dr. Parikh emphasizes that car seats may not be safe for use outside the car and that manufacturers should be more effective in warning parents of misuse and in designing seats with more stability.
For more information on the study, please visit healthfinder.gov's article. And for more general information on car seat usage please visit the American Academy of Pediatrics.