Symptom: you get the dreaded ‘Storage is Full’ error, even though you confirm that you have multiple gigs of space remaining.
Probable resolution: there’s a bug in iOS where when there’s a new iOS version and it’s automatically downloaded to your device, it chews up any/all disk space for some reason. If you delete the new iOS downloaded version, it will clear up the problem. This bug has bitten me several times and it’s annoying.