Alexander from Josie and the Pussycats (which is considered a "Scooby Clone" show, and Alexander was the Shaggy of the series) is voiced by Casey Kasem, the same person who voiced Shaggy.
This caused an animation error at one point in "The Haunted Showboat" from The New Scooby-Doo Movies (which Josie and the Pussycats guest-starred in). In one scene, Shaggy says "Me too!" to which Alexander replies "Me three!" However, Alexander replies this in Velma's voice. Immediately following this, Shaggy, Scooby and Alexander run away and begin screaming, however, Alexander screams with Velma's voice once again. Though there were many instances of characters swapping voices in this series, this instance was the longest of the series.
This was possibly due to the fact that it would have caused Casey Kasem to have two lines in a row for different characters, and the fact that Shaggy and Alexander's screams would not be differentiated from one another due to Casey Kasem using very similar voices for both characters.
Bonus points to anyone who guessed why I picked this particular episode for today's fun fact. Today's fun fact is #222, and this is the 2nd episode of the 2nd season of the 2nd series. Get it? :D