broken seats are not allowed!!
sometimes the seat actuator, which alllows you to adjust the seat forward/rearward is broken. this is ok if your seat is fixed in the upright position.
my advise, if your seat is broken, inform the crew before the aircraft takes off. they will need to call maintenance to fix it. if they cant they seat will be considered inop and needs to be unoccupied until fixed.
if the aircraft is full then the flight crew need to organise somethin(maybe ofloading of passengers)
i know this is not an ideal situation but in the event of an accident , your seat is crucial for protecting you against injury, hence its servicability is critical!!