Did you choose "Error Messages" in the "Phrase Type" drop-down menu when you added the phrase? I think in general a phrase that's used by a page should be a type that's pre-loaded by including the name in the $phrasegroups array, and phrases used as error messages should be in type "Error Messages" (which doesn't have to be included in $phrasegroups). (Or at least this is what I was able to work out by looking at the code for a bit - I'm sure someone will correct me if I'm wrong

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