Usually this is by the means of having some contradiction between the medium and the message; for example, by presenting a message in a form that inherently defeats the ostensible purpose of the message, or in a form that is fundamentally incapable of carrying the important part of the message.
Form-versus-content humour is very popular in the hacker culture. An example this is the term "SIGHEIL". By its form, it appears to be a signal from a POSIX OS, yet its message is the German Expression Sieg Heil, which is a symbol for the Third Reich.