Playing as Germany, M-R with SU.
SU attacks Poland first. Poland takes one province. I attack Poland and make them capitulate. In the process, I take over this province owned by SU. (should have asked for military access to transfer it to the SU).
SU asks me to honor MR, I accept. SU gets the states but says that I betrayed MR and cancels it. Still, MR is in place!