One line of thinking is that it's invalid not because "it isn't fair" (as you mention in another answer), but because it's impossible for a reporter at dispute round N to know whether the answer was known at market end time.
In other words, the market is improperly formulated.
If it had been the intent of a market maker to use Augur as a "bounty market", they would have been better off asking "Will [answer to NP-hard question] be known by [date X]?", specify a [blockchain Y] address as the resolution source, and the procedure by which the answer will be delivered to said address in the additional details.
The "blockchain" part is required for unambiguous timestamping, and explicit procedure description to ease verification/reporting.
EDIT: Slightly OT, but "oracle systems" in general (and Augur in particular) are well-suited to resolve temporal events, and not so well for event-less statements about knowledge.
"Is there such a word..." creates a certain ambiguity for a market trader when evaluating a market maker's intent; whereas "will a word be revealed..." (with specific instructions) less so.
Finally, the two existing markets that have (probably) sparked your question specify "general knowledge" as resolution source.
Ask anyone in the street, "Is there an English word that has md5 hash equal to
CD880B726E0A0DBD4237F10D15DA46F4?", and they'll answer "I don't know". In fact, even those who know what an "md5 hash" is likely won't be able to answer.
In other words, the answer to that question is not in the realm of "general knowledge". :)
EDIT: It has been brought up elsewhere that "general knowledge" these days means "googlit". This can be argued as a valid argument, provided the reporter doing so knows to check the timestamp... Which I wouldn't count on.
P.S. Finally-finally: the markets lack additional details, which hinders verification even by a well-equipped reporter.
Character encoding, endianness, byte padding? Definition of "English"? (British? American? Outback? Pidgin? All of these? Which dictionary? African or European?..)