I have something similar going on, except it's on the stem for a fan leaf. I first thought it was the branch at that node that had grown into the stem, but that node HAS a branch coming out of it...If a picture is worth a thousand words, how about a movin' picture?
[video=youtube;FeQx5o6rB8Q]http://www.youtube.com/watch?v=FeQx5o6rB8Q[/video]
And then yesterday I found like 3 more, on 2 different plants. They're both from a bag of some weed that was grown outdoors in humboldt last season, and I'm growing 'em in an 18 gallon DWC...But definately weird. I think maybe it was supposed to be the branch, and when it grew into the stem another one just took it's place, but I have like 4 or 5 of these sites between the 2 plants, and every node with one of these mutant stems also has regular branching in addition to these.