Paper-Thin Disguise/Fan Works

A large degree of Kirby fanfiction gives Kirby the ability to become completely indistinguishable from whoever he's taken the power/costume of, despite the obvious size differences.
Kyon manages to call Tsuruya and make her think he was his uncle Kintaro in Kyon: Big Damn Hero, even though he sounded identical to normal. He was sleeping in the same room as her at that point, though.
The incredibly useless Invisibility Cloak in My Immortal: people can see the cloak when it is in use, so basically the users are walking around with a regular cloak over their heads.
According to Light and Dark The Adventures of Dark Yagami, a naked woman can pass for a man merely by having a tattoo that reads "IM A DUDE".
In the Calvin at Camp episode "The New Kid," Larry Koopa infiltrates the camp while wearing nothing but a Mickey Mouse hat.