I've seen that happen once before on a tandem (one sided step through). The TI brought back some linetwisted, stepthroughed, drogue woven in and out of the only 3 cells able to be identified canopy, and the packer had to chop it in order to sort everything out. He everntually cleared it but must have flipped one of the risers through itself, creating a one sided step through. The main opened, flew fine, and was fixed upon landing.