Due to the total lack of sensible hero sorting and the way the heroes change places depending on what part of the game you're in, plus the hero portraits all facing the same 3/4 to the right, I've occasionally selected the wrong hero and lost the occasional PvP battle as a result.
Due also partially to red-green color blindness I've mistaken:
Solaris and Krexx
Cobressa and Viperia
Emily and Black Diamond
Icebloom and Yasmin
Any other heroes you think might be twins but were separated at birth?