Not every plane flying is in perfect repair, though the airlines are
very careful to take the problems into account. For example, when a
plane takes off, the front wheel is spinning. If the special brake to
stop it in during takeoff is not working, then the wheel cannot be
retracted
until it slows down on its own. If it is retracted while it is still
spinning the heat of the rotation may
damage the wheel or the housing. The plane can still fly, but it cannot
carry quite as much weight during take off as it could if the wheel
could be retracted earlier.
Needless to say, airlines like to avoid deferred maintenance items, but
sometimes it is better to unload a few bags rather than cancel a flight
and throw the whole system into a tizzy.