Here are some other agency ruses I've noticed recently to add to the handy top 10 list by Gerry.
I'm posting these especially for those newer to contracting.
The client wants to hire your services, but becuase your experience and skills do not match the role 100%, they will give you training when you join. For this reason, they are not able to offer the rate you agreed upon.
Not exactly going to make you fall outside of IR35 is it? Respond with the following: you are happy to pay for training as a business expense. Ask for a conference call with the client to clear this up, also to see what sort of books you might buy. The client explains there is no training, just a standard intro day or two to the project for functional requirements only.
Another ruse - the project is long term, at least 12 months. The client has a lot of other projects going on and if they like you, you will be put on other gigs, so becuase it's long term, they want a rate cut.
This trick is often used to attract you to a role. Make sure you ask the client how long they need you for to be sure. An agent says 12 months, the client says 2 months only.
There's also a new one on reference harvesting for business development.
Agent has some interviews planned. You are the second candidate. He rings you up, claiming that his professional reputation is at stake. The client has given the other candidate a technical interview and rang the agent up, protesting that the candidate's cv was total fiction so please could you give him all your references for projects on the cv so he can ring up your clients to verify your suitability for his client's project, or he may lose the customer. You might lose the interview if the client loses faith. Nothing like a bit of pressure for you to share your confidential client data :-)
Seriously, we could write a book about these beasts.