Faster, Bunny Rabbit! Kill! Kill!

Say an author submits a manuscript that you like and ultimately offer a contract. Next thing you know, you're dealing not with the writer but from the agent s/he was able to get because there's now an offer in hand. How do you feel about that?

No harm done. I’ll be significantly less ambivalent, though, if the author not only received my offer but agreed to it... and then failed to tell their new agent. Suddenly there’s an agent in the mix who thinks the advance and royalty are still up for negotiation, and is perhaps eager to prove their value to their new client.
As I said before, I don’t mind negotiating; not at all. Re-negotiating, though, shows a lack of understanding of the rules--and steams my bunny slippers.