What word describes the concept of how what is generally held as to be true, is in fact false (at least most of the time). I think a fictional story illustrates an example the best:
Tinkerbell lives in Never Land. Tinkerbell got lucky and secured a job as a fairy. Everyone knows being a fairy is the best job that exists in Never Land. However, after becoming a fairy, Tinkerbell learns that most fairies find the job terrible and it is actually really unenjoyable being a fairy.
Another example (completely made up):
Most people assume Olympic Athletes are fit, but in turns out this is untrue as a great deal of the time and to compete in the Olympics is determined more by connections than athletic ability.