Had enough devices that slowly degraded until I got stuck diagnosing there marginal intermittent faults and where promptly removed for refurbishment.
Many power transistors running hot suffereing from beta loss,
Many bridge rectifier diodes with increased leakage,
H bridges with partial shoot-through thanks to parasitics from dust buildup,
Logic gates that degraded into linear modes.
Canbus Transceivers that fail with can low shorted to ground.
As for devices that have failed gracefully,
Generally hard shorted tantalums on current limited rail, nice clear fault, generally noting damaged, and easy to diagnose.
Open Transistors, C-E open with no damage to the base driving circuit, not as common, but a pleasure to diagnose.
Resistors that creep up in value (generally old film based power resistors), they get hotter and hotter leaving a nice tell, and generally only brownouts.