It's likely you would have heard of, or perhaps even experienced, Tetris syndrome—a common phenomena that refers to a sort of bleeding effect between #VideoGames and real life, the lines between each beginning to blur after prolonged and delirious play sessions.
It gets its name from the stupendously addictive puzzler of course, but is far from limited to that game. Though I've yet to see any imaginary tetrominos, I can recall that, during the height of my Modern Warfare addiction, I would often shut my eyes to see a full, vivid Team Deathmatch game play out.
This kind of projected hallucination isn't much to worry about, health wise. In fact, a 2009 Oxford study concluded that a preoccupation with Tetris-like video games "interferes with the way sensory memories are laid down in the period after trauma and thus reduces the number of flashbacks that are experienced afterwards."
In any case, while video games certainly have their benefits, we can all get a little confused sometimes...