Portal 2's Wheatley fits extremely well as the voice of 343 Guilty Spark in Halo: Combat Evolved. This mashup takes the cake.
A Redditor has done fantastic work for science by compiling a gallery of Portal 2's wall decorations. There are more than 30 in the collection, so chances are you missed a couple as you cruised through Aperture's numerous facilities. And of course, they're all hilarious.