The worst thing is...
breathing on manual
being aware of your own blinking
being aware of the taste of your own tongue
hearing that Rick Astley is never gonna give you up
losing The Game
losing The Human Centipede Game
something else I will explain in comments.
(The Human Centipede Game is, of course, lost by remembering that the movie exists. The first time you lose it is when you discover that the movie exists. And need to go for a long lie down.)