Then there was the Teen Titans episode where Raven (Tara Strong) and Starfire (Hynden Walch) had their minds switched. The original plan was to have the actresses play each other's characters, i.e. to keep the voices with the right bodies and just change the personalities. But Strong and Walch sound so similar, and are both such excellent mimics, that Strong's Starfire and Walch's Raven sounded almost exactly like Walch's Starfire and Strong's Raven anyway. The average viewer wouldn't have been able to tell that the actresses had switched characters; it would sound like the voices had switched along with the minds anyway. So they abandoned that plan and just had the actresses play their usual characters -- Strong as Raven in Starfire's body, Walch as Starfire in Raven's body.
Still, I regret they didn't stick with their original plan. I think I would've been able to tell the difference, since I have a good ear for voices. And I would've loved to hear the actresses playing each other's roles.