Donnie's missing tooth

[funatic]

I mentioned this as an afterthought in the Monster Arc thread, and am surprised that no one else was surprised that those episodes inferred that Donnie has been missing a tooth this whole series:

Originally Posted by funatic

I'm probably the only one just now watching these Halloween episodes near Christmas, so sorry if it's been mentioned before, but did anyone notice how Donnie only had three incisors between his canine teeth after he became a vampire?? (visual aid: https://www.youtube.com/watch?v=CcB1XndrXHw) All this time I thought he had a big gap, but this suggests he's actually missing a tooth. Weird.

Did everyone else always think it was a missing tooth, or did you all think he had a gap this whole time like me?

Conversely, maybe this was just a design error that slipped through the cracks in a big way?

Let's get to the tooth of the matter.