It appears that Target has sold me an entire box of defective zip-closure freezer bags. When I went to use them this past weekend I found that they wouldn’t close, and closer inspection showed that two halves of the zip-closure are misaligned. Thus, the blue part can’t latch into the clear part and it’s not possible to close the bags.
I intend to return them and get another box, but I suspect that the entire run of them is defective.