Mirror modifier. Instead of just moving the mirrored vertices it creates new vertex on each side I have encountered a weird behavior when I try to move one or more vertex connected to mirrored ones. instead of just moving the mirrored ones it creates new vertex on each side connected to the old ones an example of the behavior I am encountering can be see in this picture.
This was a simple cube in which I tried to move an edge while the mirror modifier was active, is there any way I can prevent this behavior?
Turn on Bisect, it will cut the model in the middle and mirror it instead of copying/reversing it.