For some reason a texture has been rotated on the face of a brush until it is "stick out of" the brush face. Causes are usually, but not always: vertex manipulation, clipping the brush, carving the brush, importing a prefab, and sometimes rotating/skewing a brush with more than 6 sides - like a cylinder - without texture lock on.
Often this error can be fixed using the "check for problems" dialog (Hotkey: Alt+P). In the rare case the error isn't shown, you will need to search for the error manually.
Normal texture axes are projected nicely onto the face
Perpendicular texture axes can't be projected onto the face, and the texture cannot be applied correctly.