Ask HN: Examples of standard compliance that does not work in the real world?

From another HN thread:

RFC 5321 states: The local-part of a mailbox MUST BE treated as case sensitive

Which is absolutely impossible to follow in the real world. An onlineshop that treats Henry@gmail.com as a new customer because it is different from henry@gmail.com will have a ton of problems.

What are similar, maybe even harder to follow standards?