Sharp PC401 Optoisolator Failure
Any part can fail, however parts that are closer to the user and can be abused (knobs, input jacks etc.) or are stressed (off-line SMPS components, those carrying high currents, those that get hot) tend to fail far more often. 

Optoisolators have an aging failure mode, however that only applies when the LED is powered and is accentuated at high temperatures. Storage is very benign.