April Fools' Day: The announcement of a Magical Girl anime series Inspired By the American The Powerpuff Girls was assumed to be an April Fool's joke, since it was announced on April 1st, 2005. Doubly so since after the announcement was made, there was absolutely no new information about it until April 1st, 2006.
Playing Against Type: In the English dub of the TV series had a very surprising example of this trope with Richard Ian Cox as Big Billy (later known as Big Boy). Out of all the Gangreen Gang members RichardIanCox played Big Boy, granted he does try to do a different type of voice though. (But of course this could belong to WTH, Casting Agency?.)
Quite a few of the voice actors of the dub also provide voices for My Little Pony: Friendship Is Magic. It's oddly appropriate since that show's creator, Lauren Faust is the wife of Craig McCracken, the man who made the original show. Specific examples are Tabitha St Germain, Cathy Weseluck, Kathleen Barr and Maryke Hendriske.
Science Marches On: This is a 2006 series. Why is the major, obviously on the breaking edge of chemistry, scientist still using FLOPPY DISKS?
The prototype animation is Darker and Edgier; the characters are older, it's more action-y, and the personalities are more similar to the source cartoon.
Apparently the voice actors for the original series have expressed interest in making their own dub of this (with Craig McCracken's approval).
Concept art shows a variety of unused designs. Some of them look older than the finalized designs (one even having the girls wearing midriff bearing shirts and jackets while another uses the classic clothes with scarfs) while others are simply more animesque versions of their The Powerpuff Girls designs. One of said PPG designs are oddly sci-fi looking. A certain design for the girls has them all wearing coats, and Buttercup's hair is in a traditionally styled bob haircut. It's unknown if the girls were unrelated or not in these earlier concepts.