1. Trust in Encyclopedias
When I was a kid, if something was in the Encyclopedia Britannica (or even Grolier's), it was true. Now--thanks to Wikipedia--having "encyclopedic knowledge" of a topic isn't as impressive when there's a good chance most of what you think you know was concocted by a 12-year-old. After a 2005 study by the British journal Nature showed Britannica and Wikipedia to be equally inaccurate, faith in all encyclopedias plummeted. Britannica attacked that study's methodology as "fatally flawed," but it was too late.
Also dead: trust in studies of encyclopedias.