I have a 3 year old maxi cosi cabriofix car seat which I used for about 12 months for ds1. In preparation for ds2 I washed the car seat covers by hand, following the washing instructions on the label. The result is that the seat covers are now completely misshapen and the foam filler seems to have expanded.
Maxi cosi won't do anything as the warranty is only for 2 years.
Am
as I deliberately chose a maxi cosi for it's safety profile and the fact you could wash the seats if necessary. I can buy replacement seat covers, but wanted to warn others in case they try to wash them too!