What version are you running?
0.7.x through master
What steps will reproduce the problem?

Create a review request for a repository.
cd into a different repository configured on the same server.
rbt post -r <id> with the ID from the first review request

What is the expected output? What do you see instead?
Instead of denying the post because the diff is for the wrong repository (and therefore won't apply), it allows the post. The uploaded diff in Review Board will not be renderable.