What is the difference between Digital Input (S/PDIF) and Headset Microphone
Solution 1:
It might be a bug occurring with USB devices: https://bugs.launchpad.net/ubuntu/+source/alsa-lib/+bug/1002952
If you have a USB headset, or other USB device that does not have any S/PDIF output, but yet sometimes there is an extra device called "Digital Output (S/PDIF)" for that device, you're suffering from this bug.
This bug is only for USB devices - if you have an invalid digital output for some other type of device, please file a separate bug instead.