Tough one - it would, as other people have said, very much depend on what the orginal issue was and what the extras were - e.g. if your cat goes in for a thyroid removal and comes home with no lymph nodes as they all had tumours too, you'd probably be grateful.
Our vet gives us a consent form to sign, listing exactly what he intends to do, but then he also gets a vn to phone if he finds something new or if he can't do something (when our dog was spayed I got a phone call to say they couldn't trim her nails as there was nothing to trim, bless them).
If you sign a consent form and it says something about "other such surgery as the vet may consider necessary", you're probably stuffed - unless you can prove that the surgery wasn't necessary and your vet's incompetent.
Do let us know what the outcome is, and more details when you can (legally etc). Just don't forget to include words such as 'alleged' so that you're covered against the aforementioned libel and slander...