Today I realized that I forgot the password to decrypt one of my hosts encrypted using Luks. Luckily it was still running and root-access still granted.
As you may know root can do anything, even reading the masterkey of your unlocked Luks-devices.
By using
dmsetup table /dev/mapper/supercrypt --showkeys
you can get hold of the masterkey.
Since you only unlock the masterkey with your password, this information can be used to modify every slot.
In my case adding a new password to a new slot saved me from losing the content of this host:
cryptsetup luksAddKey /dev/sdc --master-key-file <(dmsetup table --showkey /dev/mapper/supercrypt | awk '{print$5}' | xxd -r -p)
Found in the redhat knowledgebase.