After several out-of-scope-revisions, I asked the client to put in place a set of final acceptance criteria. The client agreed, drafted it up in writing, and had me implement the final changes according to the detailed specifications they provided.

Client: We just want you to make a few more changes. It’s not quite what we were looking for.

Me: Sure, I’ll write up an estimate based on the amount of time and effort that should take.

Client: Well, we were hoping you could roll it into our ongoing changes.  After all, we never technically signed off on the last set of changes. 

Me: You wrote them though. 

Client: Maybe, but I don’t remember signing anything.