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PostelsLaw
Postel's Law is about communication protocols. It states "be lenient in what you accept, and strict in what you emit".
Postel's Law is frequently misinterpreted to mean that servers of a protocol should accept input that is invalid according to the protocol's specification. That's not what the law means, and as JoeArmstrong points out here it's a bad idea to accept invalid input. Rather, Postel's Law means that the specification of the protocol should tolerate variability or triviality in the input. E.g. extra whitespace between fields should be allowed; an empty list of commands to execute should simply result in nothing being executed, etc. This makes the protocol more human-readable/writable. It can also save code in both the client and server, because there are fewer CornerCases to deal with.
The misinterpretation of Postel's Law is called out explicitly in TheArtOfUnixProgramming (TODO: find quote).