Strange, I have never seen mindfuck be used as the other two write-ups in this node use it, but I have seen this node linked for a different meaning.
A mindfuck is a work of art (book, film, tv show, maybe even a painting, I suppose), which messes with the mind of its audience. It challenges their worldview. It makes them question reality. In general, a mindfuck makes the audience think about the world in a different way.
Now, the description I have provided so far is not as negative as one would expect a word derived from an expletive to be. While a mindfuck may widen the viewer's view of the world, it does so in a shocking or forceful way.
The most well-known example is probably The Matrix because it makes the viewer think "Wait a minute, I could be in a Matrix." Another popular example is Fight Club, which other nodes have covered well.
Getting away from movies, Pseudomancer has compiled a nice list of books that will induce a mindfuck.
This definition could use some work.