Bug: !!Missing!! - Conv_A1C1CainIntro:MenuLab

I was playing Single Player on my second character, a Barbarian, when I noticed an odd chat bubble after speaking to Cain. In the chat bubble it displayed "!!Missing!! - Conv_A1C1CainIntro:MenuLabel". This was immediately following the "Talk to Deckard Cain" step of the "The Legacy of Cain" quest. The quest continued to progress normally, and Cain proceeded through the secret exit.

I was unable to re-create this issue on any subsequent play-through.

Screenshot: http://dl.dropbox.com/u/3688295/BUG%20REPORT%20-%20Cain%20Text.bmp