Steps to reproduce the issue

Tested with 4.2.7-RC1.
Global Configuration -> Cache.
Set System Cache to ON - Conservative caching. Cache Handler File.
Set Path to Cache Folder to \cache, and save. The Path to Cache Folder field is emptied, and administrator cache is still in use.
Set Path to Cache Folder to /cache, and save.

The value /cache remains in the Path to Cache Folder field.
At this point the user probably give up and select System Cache OFF and save.

The user is locked out. Both front-end and back-end gone.
By enabling System Debug in configuration.php the user can gain access again and empty the Path to Cache Folder field.

Expected result

Error message yes, but not a system crash.

Actual result

Nasty trap.

System information (as much as possible)

Additional comments

Same error from another user:
https://forum.joomla.org/viewtopic.php?f=808&t=999115