You may run across this error. I inadvertently did. It happens when the content database upgrade process fails expectantly and then the Mount-ContentDatabase won't execute the upgrade again because it thinks it is already upgraded. Unfortunately, this is a catastrophic failure and will require you to restore your Content database and rerun the content database upgrade.
How did I run into this? Well, I opened many powershell windows to be "multi-threaded" in my upgrade and the upgrade code didn't like that at all. It complained about a shared log file and killed off half the threads. I guess you shouldn't fire that off more than one instance like you could in 2013 and 2010!
Be sure you always backup your databases before you upgrade in case you need to rollback!