c.coyle
Fighting the Dunning-Kruger effect.
There is no failure mode for resistors to drift low. The core conductor becomes compromised by a number of possible mechanisms and this never results in increased conductivity, only reduced conductivity (there are some materials which increase conductivity under some of scenarios but they are not used for making resistors) and hence increased resistivity.
I occasionally come across a resistor that tests low in-circuit, so you have to lift a leg to disconnect any parallel resistance if you want to make sure it is not really low. I have very occasionally come across resistors that have been slightly out of tolerance on the low side when tested out of circuit. I have always assumed they were low when they rolled out of the factory, or maybe mis-marked, which is effectively the same thing.