I remember watching a training video where the PF arms the approach PB at something like 6000 feet(visibility was very low) and expected the A/C to intercept LOC and come down with the glide, but it never intercepted the glide slope, and he overshot the runway at 6000 feet..
Not sure why or HOW that happened..
If the aircraft is in a mode that maintains altitude (E,G ALT or VNAV ALT) and already above the glideslope, it never gets to the glideslope in order to intercept it, so the aircraft maintains level flight.
It happens quite often when given tight vectors to intercept the ILS.