Ooh, here's something I've never seen before. I was updating busybox on all the ROMs. No probs. When I flashed against my primary I guess the installer went looking for installed root, found a directory with an unzipped SuperSU, and decided to install there. That was unexpected but funny at the same time.
LG G3 D851, Mahdi v2.9 ROM (4.4.4), microG (NoGApps), MultiROM, XDA Legacy