He actually makes an appearance in Kevin 11 as a hologram. It is here: http://www.gogoanime.com/Ben-10-season-1-episode-7 at 14:14 --Evolved To'kustar 07:30, May 11, 2011 (UTC)
Just to add to what Evolved To'kustar said: It's when Kevin is trying to crash two trains together, while Ben is searching the Omnitrix for Heatblast, right before Fourarms' hologram appears.
Blaziken rjcf 07:39, May 11, 2011 (UTC)
Nevermind. Comparing the hologram from the first episode of Ben 10 and the one here, it's obviously Diamondhead.
Blaziken 21:28, June 5, 2011 (UTC)
2.1 Updated with new information Edit
It seems that when XLR8 is spinning one of those goons in the arcade in a whirlpool, he makes a wierd screaming sound that I apparently heard all over YouTube, especially with memes. Can someone confirm if that is true or not? If so, it should be added to Trivia.