I am pretty sure it's a defect, and not only with your 220 back. I have two 120 backs, and with one of them (or maybe both?) I have been able to inadvertently trip the shutter on two ocassions with the dark slide still in - and the dark slide was in the right way, not rotated 180 degrees. On both ocassions I detected the problem before it was too late. I have not been able to intentionally trip the shutter with the dark slide when I tried later on.
BTW, I am referring to the SQ-A, which is what I have.