乗り過ごす means forgetting get off the train,bus,and so on.It is cause of the person has no careful.so, persons can not notice the station where the person should get off.
however,
乗り越す means has two meaning.
one is like upper sentences.so,some persons forget a station where they have to get off.
and,more over,another meaning is cause of On purpose.
they know the station where they should get of.but,for example,a person get a mail.
so he have to go to more next station.
please figure out from my poor explain:)