Never trust Wikipedia, use the encyclopedia
The core idea stands, but key numbers or details have changed.
TaughtWikipedia is written by anonymous strangers and is unreliable by design. Use Encyclopaedia Britannica in the library, which is written by experts and fact-checked.
NowA 2005 Nature study found Wikipedia's science articles had roughly comparable error rates to Britannica. Britannica ended its print edition in 2012 after 244 years. Wikipedia is now standard reference infrastructure, though it remains a tertiary source you cite through, not from.
What actually happened
Nature's 2005 comparison had experts review 42 paired science entries blind. It found roughly four errors per Wikipedia article against three per Britannica article, including four serious errors in each. Britannica disputed the methodology forcefully and Nature stood by it. Either way, the gap was nothing like the one being described in classrooms.
What changed since is that Wikipedia grew defenses: mandatory sourcing, protected pages, bot patrols reverting vandalism within seconds, and rising quality on high-traffic articles. Meanwhile Britannica stopped printing in 2012, and the 32-volume set that anchored a thousand school libraries became an artifact.
The part of the teacher's rule that survives is the important part, and it was never really about Wikipedia. You do not cite any encyclopedia in serious work, Britannica included, because encyclopedias are tertiary sources. The correct move then and now is to use it as a map, follow the reference list to the primary literature, and cite that. "Wikipedia is garbage" was wrong. "Do not cite tertiary sources" was right, and stated poorly.
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Sources
Who was taught this
Still standard through 2015, so anyone who finished school between 1950 and 2005 learned the earlier version.