Re-treatment due to missed Mb2 is probably the most commonly referred procedure that I do. And almost every time, I leave the other canals alone. in this case, the referral did endo and placed a new crown, after which the patient had pain followed by a sinus tract. G.p cone tracing of the sinus tract led to the mesial root. We managed to do this with the very conservative opening, thereby preserving the new crown. (in hindsight, i probably should have left the mb1 alone)