In a MilSpec project, one performs DriveByAnalysis, and then one produces several shelves of documentation discussing what the program must do and how it will do it. (This is in remarkable contrast to how military battlefield planning works; consider it a welcome vacation from such angst.)

Reviewers of these shelves often include those who have risen through military ranks.

Partly to keep them occupied and feeling useful, and partly to ensure they really do read TheAlmightyThud, the analysts often spike their sample. They add both obvious and devious problems to the specifications. Documentation written in isolation from code, as we all know, is otherwise deceptively simple to concoct without any errors that anyone could detect without actually attempting to translate them to code.

These fault insertions are called "HairyArm''''''s" in some circles.

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''Why are these called "hairy arms"?''

Because they are designed to be caught by those who rose through military ranks. To start, you need a bit of brawn.

''Also, the meaning and tone of the above "definition" is not clear.  Are these hairy arms really deliberate insertions of faults, or does it just seem that way to the unfortunate people who have to deal with them?''

They are deliberate DefectSeeding. They make the brass feel useful, and they test how closely, or indeed if, folks read the documentation.
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See SpikedSample.
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Is this related to the Van Halen "Brown M&M" rider? (http://www.snopes.com/music/artists/vanhalen.asp)

CategoryBug (intentional)