I think people have generally cottoned on that it's a bad idea to cite Wikipedia when trying to establish something as fact. But let this gem from Britannica Online serve as a reminder that the problem goes well beyond the fact-police's favourite whipping boy:

From the definition of "tautology": Thus, "All men are rational" is held to assert with regard to anything whatsoever that either it is a man or it is not rational.

This is, of course, completely wrong. The second statement is the converse of the first. Let me make two quick points in Wikipedia's favour:

• If this was on Wikipedia, I could simply have fixed it instead of writing about it.
• The Wikipedia article on Logical Converse has a nice long explanation of why this is an error.