When I want to point out to my students that getting the right result is not enough, I like to show them the example:
$$\frac{16}{64} = \frac{1\hskip-.1cm- \hskip-.4cm{6}}{-\hskip-.2cm{6}\,4} = \frac14.$$
This example works well in some of my classes where the error is just the right level of "stupid": The students feel absolutely safe from making this error, and they also do not feel that this is a "kindergarten" example too far removed from their level of mathematics.
But I would like to have some more examples like this for students with less and with more knowledge about mathematics.
What are some good simple preferrably amusing examples that getting the right result is not enough?