Yellow Batsuit

The now infamous Yellow Batsuit is a suit of armor presumably worn by Batman at some point between the end of Batman: The Animated Series and the beginning of Justice League. The creation, use, and purpose of the suit has been a topic of endless debate and James rage.

History on Screen
The Yellow Batsuit has been seen exactly one time in the existing entirety of the DCAU. It appeared in the background of the Batcave during "Starcrossed", the series finale of Justice League.

Appearances
The suit bears a striking resemblance to Batgirl's suit and also appears to be a yellow version of the suit worn by Terry McGinnis in Batman Beyond.

While the suit does not appear again, a child wearing a black t-shirt with a yellow Batman Beyond style emblem can be seen in an issue of Justice League Adventures.

Batman Beyond Prototype
The first theory as to why this suit exists is that it is a prototype of the suit eventually worn by Terry in Batman Beyond. This theory is reasonable enough until you ask why it would be on display in the Batcave.

In issue six of Justice League Beyond, a companion webcombic to the Batman Beyond series, Terry's suit is damage and Bruce provides him a prototype to use while the main suit is repaired. Spoiler alert, the suits are not the same.

In the Batman Beyond episode "Disappearing Ink", Bruce shows Terry another prototype. This second prototype is much more mech-like, thus meaning if the Yellow Batsuit is indeed a prototype, Bruce is arbitrarily displaying a 3rd round prototype alongside his former partners retired suits.

Artistic Mistake
The second, and most likely, theory is that the Yellow Batsuit is simply an artistic mistake. As explained by James in The YELLOW BATSUIT - EXPLAINED?, the Batcave we see throughout Justice League and Justice League Unlimited is the same Batcave style used in Batman Beyond. Despite the time gap between the seasons canonically, what most likely occurred was someone in the art department realizing they left the Batman Beyond suit visible in the scene & quickly recolored to something different before the show went to air.

Legacy
James will never forget this suit and now, at the time of creating this article, neither will 454,433 other people because he screamed about it for 9 minutes, 41 seconds.