As I was preparing to install a 3-gang old-work box I discovered a random cripple stud whose only purpose for existing is screwing up this project for me:
In the bottom right is the corner of a 2x6 which is flush against the back drywall and against a stud to its right and extends all the way down to the sole plate. Using an inspection mirror I determined that there is nothing attached to it or passing through it anywhere along its length, and there's nothing on the other side of the wall either (not even any sign of drywall patching).
How can I trim off the corner of this stud so that I can install my old-work box? (The actual area of interference is about 1" high and 1/2" wide.)