Indian Summer (OAV)

takaya's room in episode has a poster of Dokuro from "Club to Death angel Dokuro-chan", and almost completely off-screen is a poster of Shana from "Shakugan no Shana" but only part of her coat and hair is visble. In episode 3 a different Shana poster is fully visible.

In ep. 2, some of the maids participating in the contest include Mariel from Hanaukyo Maid Team, Multi from To Heart, Mahoro from Mahoromatic. Although the maids have the same masters, their names are not the same (i.e. the "Multi" maid is called "Luchima").

In the 3rd episode at about 10 minutes or so when Murase goes in his room to find Yui on the computer, you can see a poster of Shakugan No Shana on the wall.

As seen at the end of first part of Episode 1, Takaya has two "Ichigo Marshmallow" posters.

First part of third episode parodies Maria-sama ga miteiru - even "onee-sama" character has identical design to Ogasawara Sachiko even voice actress is the same person.

In episode 2, at the conclusion of the fight sequence against the octopus, Ran and Sumire are blasted into the sky and disappear in an homage to Team Rocket from Pokemon.

In episode 3, the teacher (at timing 5:59 to 6:25) is wearing the same clothings as Mizuho Kazami from Onegai Teacher