The 5th episode of both season one and Träumend are full of incredible hijinks.
Shinku finding out what a bathroom is used for, slowly blushing and then locking herself in her case qualified for sure.
Kanaria is led to believe that the girls were going to use Shinku's cookies to knock her out then steal her Rosa Mystica. She goes on to try and gloat evilly about how she saw right through their "plot" before Suiseiseki intervenes.
Kanaria: You really thought I, Kanaria, the brainiest of the Rozen Maidens could ever possibly fall for such a—(Suiseiseki shoves a cookie in her mouth)
Suiseiseki: SHUT UP! (pulls a massive tray of cookies out of nowhere and pours them down Kanaria's throat) Okay, open wide!!!!
In the sixth episode of the 2013 anime, Shinku and Suigintou get into an incredibly juvenile fight over a removable shower head. The very next scene has them both sulking, wrapped in towels while they wait for their clothes to dry.