We were well into the production of a client's web site when he suddenly requested significant functionality changes to the site. As per the terms of our signed and agreed contract I gently indicated that the changes would have to be rescoped and as such would be subject to supplementary charges as they were a variation to the project specification that he had agreed to at teh outset.
Client: I haven't signed anything
Me: Yes you have, document ref XXX and dated XXX, I have your signed copy here.
Client: No you don't
Me: Yes, I do shall I fax it to you?
Client: Er, yes!
[Document faxed over]
Me: Is that your signature?
Client: I'm not saying!
Me: What do you mean....?
And so it went on. We eventually pulled his site from the web until the situation was resolved, whereupon he eventually conceeded that it was his signature.