The author of one of the most famous lines in video games in the past 15 years — The Cake is a Lie — is a briefly visible Easter egg in Lego Dimensions.
It took a lot of hunting, but Doug Rattman — whom Portal obsessives recognize as the Aperture Science programmer who escaped GLaDOS' extermination of the Enrichment Center — appears in the game. He is not a playable character but, much like his presence in the original Portal, he lets you know he's around.
YouTuber Disney Dan shows you how to spot Doug in the Aperture Science Enrichment Center level of the Lego Dimensions Portal 2 expansion, in four separate instances. The module launched in September 2015. Doug always manages to disappear before you can get too close.
In the Portal canon, Rattman is the one who doubts that the "morality core" installed into GLaDOS can truly stop her from becoming the sociopathic tormenter she is in the first game. As players navigate the test rooms and ultimately creep behind the scenes, they discover evidence of Doug's existence, mainly through his cobbled-together sleeping camps, graffiti assistances, and insane-sounding scrawls. "The Cake is a Lie," is his most famous.
The Half-Life wikia has much much more on Doug's backstory, if you're interested in his importance.