Dunning–Kruger effect

From wikipedia:

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”. They therefore suffer an illusory superiority, rating their own ability as above average. Stephen Colbert, a talk show character played by actor Stephen Colbert, exhibits a confident and unaware ignorance typical of the Dunning-Kruger effect.

If you've never run across this you're a very lucky person.
Or you may be suffering from the effect yourself...