On the other hand, I'd much rather a client pitch a great idea that's technologically infeasible than just come up with something spur of the moment that they deem possible. If you start the conversation at, "This mechanic may not be possible, but the rest of the game is great," then you can discuss how to change the mechanic or maybe an engineer will have some brilliant insight about how to do the impossible. At the very least, you're starting with your best and working towards the technological compromise rather than starting with something that's already a compromise and may not reflect your best thinking.