The ones near me get confused. If there is a car behind you going slower or faster it sees both and the speed changes. Good to do when your doing 17mph, and the cars are doing 30mph, it goes from 17 to 30 to 17 within seconds, and then just turns off.
Not really tried that hard to set one off yet, keep meaning to though, i always forget they are there and cant get up to speed fast enough.
Not had a speed camera go off, but i have been through a 30mph one doing a tad more, but it hasnt gone off, i think it could be turned off though.