Insert measure at beginning of system generates extra undeletable key signature.
(1) Create a new score with a key signature other than C major.
(2) Insert one measure while the first bar of the second row of staff is active.
(3) Insert one measure anywhere on the first row of staff. *
*If there are notes on the staff, more measures may need to be inserted in (3), in order to make a measure move to the row underneath, and cause the problem.
The key signature is repeated on the second measure of the second row of staff. The same thing occurs if you do the same steps on any rows of staff next to each other.