I'm in the middle of several fairly large spec-writing projects at the moment, so this year's April Fool's RFC 6919 seemed particularly apt:
The key words "MUST (BUT WE KNOW YOU WON'T)", "SHOULD CONSIDER",
"REALLY SHOULD NOT", "OUGHT TO", "WOULD PROBABLY", "MAY WISH TO",
"COULD", "POSSIBLE", and "MIGHT" in this document are to be
interpreted as described in RFC 6919.
I briefly considered making use of this and waiting to see if anyone noticed. So far, I have resisted the temptation, and am sticking with RFC 2119.