The other thing about pre-arranged trades is that regardless of teams just backing out (which absolutely does happen), when you don't have a deal in writing and approved by the league, it leaves room for misunderstandings.
The best example I can think of is actually in the NHL, where the B's and Leafs had agreed to a trade in principle, that fell apart before they made the trade call, when both sides realized they each thought they were getting the other teams first round pick in it and keeping their own.
And I am willing to bet those type of things happen a decent amount, when guys basically hear what they want to hear, and talk past each other.
So, if you are going to make moves (or draft a player) based on a move that cannot be consummated yet, you really do so at your own risk.
Just ask Steve Kerr, who told a story the other day on NBATV about how he passed up Dru Holliday because he thought he had a deal agreed upon to get Steph Curry. And when that fell through, he was stuck...and now he is on NBATV.