Gell-Mann Amnesia

The Gell-Mann amnesia effect is the tendency of people to critically assess media reports (or, I would add, random people's opinions on the internet) about subjects with which they are familiar, yet continue to trust those sources in other areas.

It's a similar concept to Erwin Knoll's "law of media accuracy," which states "Everything you read in the newspapers is absolutely true except for the rare story of which you happen to have firsthand knowledge."