This is an issue where the bbcode is broken in the poll options section when using preview.
I had a hard time recreating the issue but finally I realized it ONLY happens when you have everything filled out except for the 'Poll Question". I filled out the post subjest, the post itself, and the poll options but not the poll question and this error happens. It seems to only happen in this situation.
The input is:
When you hit preview, it turns into:
Again, this only happens when everything is filled out except the poll question.