Bloody GIR is a picture of GIR covered in blood (hence the name) that the producers snuck in, because Jhonen wanted a GIR covered in blood in an episode, but Nickelodeon prohibited this action. Since the image was already drawn, Jhonen along with J. Bondy, Chris, and Rikki Simons (color artist on the show and voice of GIR), decided to sneak in frames of bloody GIR in about just about every episode after the refusal.
This image can only be seen in frame-by-frame viewing. Otherwise it will be a flash, or, in some cases, blend into the movement of the animation.
It is believed by many fans that this image caused the show to be cancelled. This was not the case, as Nickelodeon found out about the image AFTER the show's cancellation. The true reason behind the series’ cancellation was the lack of viewers.
Guy1- Whoa, I'm pretty sure I just saw a little bloody guy in my head.. Am I insane?
Guy2- Nah, thats one of them little Bloody GIR things Johen hid all over the fucking place.
Guy1- (Ignoring Guy 2) SQAHHHHAAAAAAAAAAHHH!!!!!!!! MADNESS!!! MADNESS!!! AHHH AHH AAAAAAAAAHHH!!
Guy2- Whoa, just chi-
Guy1- SQAAAAAAAAAAAAAAHHHHHHHHH!!!! AAAAAAAAAHHH!!!! AHHHHAAHHHHHH!!!!
(Proceeds to gouge his own eyes out.)