It may be fun, but it still bugs me, so that's why I posted this discussion. It happened one time I was doing Super Juggernaut Fanclub - I did it and then I sold the tower while the ability was still active and the ability remained active forever. It must be another one of many glitches, but it is kind of a gamebreaker if you ask me - Super Monkey Fanclub which works for unlimited amount of time.
For the Flash version of BMC, I've confirmed that it's guaranteed to work if you sell the most recently built of the transformed Dart Monkeys. So don't build more than 10 (including the Fanclub Monkey itself), and make a mental note of the one you built last. Then activate the ability, and sell that one. Voilà, 10 Super Monkeys that can be upgraded to pierce 18 or 100 for really cheap. Now you can build the next batch of 9 dart monkeys and remember the last one again, or you can sell the Fanclub Ability monkey.