Resistor Failure Modes
lead damage -- lead would be the wire or a connector what is attached to the resistor when it is manufactured .. it is broken off or damaged to the point that it affects the resistor performance (broken trace on a PCB is not resistor failure)

drift -- is the undesirable change in resistance (even though it may be predictable) ... what causes it does not really matter in this context ... a thermistor value change due to temperature change is not drift ... but a difference in resistance for the identical temperature is drift

binding/sticking --- variable resistor adjustment mechanics not working smoothly ... there is a possibility that a resistor expands as it gets hot and it may have a mechanical device to adjust for the expansion (grasping at straws there)

contaminated --- foreign substance on resistor affecting its properties (wirewound resistor that has a splatter of solder on it)

seal leakage --- resistor may be rated for damp environments because it is inside a sealed jacket and the jacket has a crack in it. it may also be immersed in a dialectric liquid for heat dissipation