I backed up a database:
BACKUP DATABASE MyDatabase TO DISK = \'MyDatabase.bak\' WITH INIT --overwrite existing
And then tried to restore it
This may be fairly obvious, but it tripped me up just now:
If you are taking a tail-log backup, this issue can also be caused by having this option checked in the SSMS Restore wizard - "Leave source database in the restoring state (WITH NORECOVERY)"