Taylor, then lets assume he never mentioned this was from the Professor Test. ;D Plus, it's not like everyone gets the same Professor Test questions for each test.
Looked through the Compendium, the only thing I could find was this:
The Compendium said:
Q. Can you use Warp Point if you and your opponent both have only one Pokémon in play each?
A. You may use Warp Point if either player would be affected by it, but if NEITHER player is affected you cannot use it. (Jan 4, 2007 PUI Rules Team)
Other than that, I would say any other time when you cannot play Trainers, such as when Kabutops MD is active on your opponents side, or if your opponent flipped heads on Vileplume LA's attack.