Confused and amazed

So I'm playing my low level sorc when I notice that she is causing poison damage when using her off-hand

Battle Dart. I look close and it doesn't say anything about poison. Cool, extra stat. I swap to a

Throwing Knife just to verify and it does the same thing.

Umbral Disk???? I swap to

Pelta Lunata and same response.

I have to find out what is causing this to happen. I go down to Act One. I can't duplicate the issue. I go back to Act Two,

Canyon of the Magi, nothing. Huh. It happened when I was in

The Arcane Sanctuary before. I go back there. Poison every time.

Anyone else notice this?