Ah yep, I've experienced the "hurry up before the it disappears" thing you mentioned - They're almost always bluffing though.
One time, I kept telling the recruiter that I needed more time to consider her offer and then after 1 week she called me and blatantly told me "Sorry someone else took the contract, you had to be faster".
I knew she was lying so I came back to her 1 week later and told her that I would take her contract if it was still available (even though she had told me it wasn't the week before) and she offered it to me on the spot and made up some excuse as to why it was suddenly available again.
I don't blame them though. They serve the company first and you second.