That is very odd. It's especially odd that none of the other black lines in the image are experiencing the same effect. I would expect that if it were something like a slippage of a plate or inkjet (or whatever they use to print these shirts), it would affect all instances of the same color, but here it affects only the text.
I'll pass these along, but I think unless there's a whole lot of other people with the same problem, it won't have been anything in my artwork that did it. More likely a fluke misalignment or something. If you want to return it and order a new one, it probably won't have the same issue.