It's not a hallucination, but when I play Guitar Hero/Rock Band for a while, and look at another screen, such as a different tv or laptop right after, my eyes are still adjusting to the notes sliding at me, and the other screens start distorting.
It's sweet.
Also, I think there has been one point where I was driving down one of the streets in my town, and I could vividly imagine making it into a sandbox horror game, because I had been playing Saints Row 2.