Probably the most famous example is one of the optional settings provided in OOXML. The setting is called "useWord97LineBreakRules", and it specifies to use the line-break rules that were used in Word '97 for East Asian documents. Much like the previous examples, this is of course impossible for anyone else to do, as no specification of these rules is provided. In fact, the OOXML standard even warns implementors not to implement this:
Listing 1. The OOXML standard's guidance for useWord97LineBreakRules
[Guidance: To faithfully replicate this behavior, applications must imitate the behavior of that application, which involves many possible behaviors and cannot be faithfully placed into narrative for this Office Open XML Standard. If applications wish to match this behavior, they must utilize and duplicate the output of those applications. It is recommended that applications not intentionally replicate this behavior as it was deprecated due to issues with its output, and is maintained only for compatibility with existing documents from that application. end guidance]
This guidance is excellent. Given that there is no specification available of this feature, and it is deprecated, it makes all kinds of sense for people not to implement it. But wait; if it shouldn't be implemented, why is it in the spec? Compatibility with existing documents is not a reason to add a feature to a standard aimed at interchanging data; users are worried about whether their text can be opened at all in another program, not whether every line break is in the exact same location!
This feature is in the spec because OOXML is not a document interchange format; it's a careful, bit-for-bit, replication of Microsoft's historical binary formats, wrapped up in angle brackets.