In developing problems that stimulate classroom discourse I have tried to present to my students problems that may require a 'counter intuitive' solution. In a problem such as, "Mary had saved three times as much as Tom. If Mary had saved $75, how much had Tom saved?" students who look for key words only might focus upon the '... as much as..' and think of multiplying rather than adopting a different strategy (division or repeated subtraction for example.
In the classroom discussion following time to arrive at a solution the role of the teacher is first to invite a representative from the group to explain the solution and the strategy used to obtain the solution. As I teacher I then encourage others to challenge the solution or the strategy (or both) by supplying proof that either is incorrect.
Of course this is a simplified example and in the classroom many of the problems I've presented are more compelex than the above.