I've never witnessed kickbacks. I haven't heard any members raise kickbacks, so I can't speak to that. However, it's always a possibility.
Again, that's why you have a code of ethics, that's why you have internal rules and that's why you monitor transactions to the best of your abilities. You can never eliminate risk, but you can reduce the risk, you can mitigate the risk and you can ensure that proper controls are put in place to reduce the probabilities of things like this happening.