Ok, it has even a name:
„The Dunning–Kruger effect is an example of cognitive bias in which „people reach erroneous conclusions and make unfortunate choices but their incompetence robs them of the metacognitive ability to realize it“.[1] They therefore suffer an illusory superiority, rating their own ability as above average. This leads to a perverse result where less competent people will rate their own ability higher than relatively more competent people. It also explains why actual competence may weaken self-confidence because competent individuals falsely assume that others have an equivalent understanding. „Thus, the miscalibration of the incompetent stems from an error about the self, whereas the miscalibration of the highly competent stems from an error about others.“
http://en.wikipedia.org/wiki/Dunning%E2%80%93Kruger_effect
found via http://www.rackblogger.de/2009/08/20/dunning-kruger-effekt/