Light switch/light problem
That is simply a mis-wired 3-way. 

On each switch you'll have a "common" terminal and two others. Typically a black screw and two gold. You need to confirm that the "travelers" are on the gold screws. Look at this diagram to see what I mean.

In this case the black/red pair are the travelers, but could be any colors depending on how it's wired. Just keep in mind, the same two wires must be on the travelers.