We had an issue when upgrading to 8.0.0 where the upgrade of the database failed, but the configuration was still upgraded leaving QB in an invalid state.
The first time we tried to run the upgrade it turned out that while we had updated the wrapper.conf in our installation to point the java command to a JDK 8, the upgrade command uses whatever the default java command is on the machine.. A couple of our internally written plugins were written against JDK 8, and the upgrade ran with JDK 7, so it threw an UnsupportedClassVersionError. Unfortunately, the upgrade continued and did not report failure. When I tried to start QB after the upgrade, I received an error that the database was at version 94 and needed to be at version 98. Thankfully the upgrade scripts take a backup of the configuration and I was able to roll back.