Throughout the game, the word "additional" is repeatedly misspelled as "additonal".
Here it is in an Oathbound Paladin power:
Here it is in Control Wizard feats:
Here it is in a companion's Active Bonus:
Here it is in Bilethorn Enchantments (Your foe takes an additonal):
I don't know what kind of development environment Cryptic uses, but I assume that you can do a (case insensitive) search of your codebase for "additonal".