Goku Sayonara Zetsubou-sensei (OAV)

In the final shot after the credits, Itoshiki is briefly pictured in front of a red mecha, wearing a military jacket. This is a reference to Char Aznable from the original Mobile Suit Gundam series: this particular jacket belonged to Char, who was also fond of piloting red mechas. The mecha on the picture additionally wears Char's signature mask.

Starting with Goku SZS, and continuing through Zoku & Banigachi, there are a series of percussion notes at the end of Chiri Kitsu's phrases and sentences. This is almost like a homage to the old "Phonetic Punctuation" bit by the old comedian/musician Victor Borge.

In episode 1, Harumi dresses up as a Red (elite pilot) of ZAFT army from Gundam SEED for the Halloween party. "Dea x Yza" and "Ath x Yza" she asks Itoshiki about refers to "Dearka x Yzak" and "Athrun x Yzak", two popular yaoi pairings of Gundam SEED characters.

In episode 1, around 19:26 in, the Indian man mentions Krishna, Lakshmi, Ganesha, and Shiva. These are all deities from Indian mythology.

In Episode 1, multiple anime's parodies are showen: Lucky Mota (real name Lucky Star), Clanoad (real name Clannad), Shakugan no Shana and Melancholy of Haruhi Suzumiya.

in episode 1 at around 12:23 there is a poster on the chalkboard that has EF written on it. could be a reference to EF- a tale of the two

in episode 1 at around 17:24 the camera pans across guests for the Halloween party. Most of which if not all are cosplaying different anime characters like yuno, hiro, sae, and miyako from Hidamari sketch, asuna from Negima!, Kagami from Lucky star, Usagi Tsukino from sailor moon, hei, and yin from darker than black, Goku and vegeta from DBZ and many others.

In episode 2, a sign behind Maria at 23:40 sys Maria+Addiction, which could be a parody of Maria Holic.

In episode 2 at 9:25, there is a poster of Inu-Yasha in the background.

In episode 2 at 9:52, Nozomu mentions Negima.

in episode 2 at around 10:36 Clannad Merchandise is shown in the background

In episode 2, during Chiri's interrogation scene, a wanted poster of Zettai Karen Children's Kaoru Akashi can be seen on the wall. It reads, "Karen Itoshiki," parody of both Zettai Karen Children's title, as well as the protagonist of Sayonara Zetsubō Sensei Nozomu Itoshiki.

In episode 3, there is a sign at 10:28 that says Sailor Moo-, which is obviously Sailor Moon.