Predictive features of the EGPWS can be overridden by use of the TERR SYS OVRD button or by a system failure. TERR SYS OVRD may be selected on an approach to a runway that's not in the EGPWS database which would return a warning as the aircraft would think it's flying towards the ground not realising there is a runway there.
In either of these cases little or no warning may be given of impending terrain impact if the aircraft is being flown towards the ground in a stable approach configuration and there is no ILS glideslope available.
(paraphrased for brevity from the OM).