What would happen if the Listener wronged someone so intensely that someone put out a contract on them without knowing who they were.

Would the Night Mother be all "Uh, no can do?" Or what?

Highly doubt the Night Mother would even pay attention to them. But suppose it did go through, the Listener is the first one to find out. What are they supposed to do? Assassinate themself? The Listener is probably going to stay alive even if the Night Mother bothered to tell them about the contract. The contractor might be killed though.

Perhaps it would depend on the transgression?

Though I can't imagine any of my characters doing something serious enough to warrant having their subordinates turn on them at the Night Mother's behest.

How could any of the subordinates even find out though? Mephala only speaks to the Listener. Besides, she would much rather watch the Brotherhood destroy itself than intervene. We saw as much in Oblivion's questline. She'd never directly command anyone to kill a backstabber to the guild, let alone the Listener.