On Fissure of Woe, I have had 3 of these skill challenge bugs so far.
Beetletun(Queensdale) – Challenge the guy, he’s laying dead on the ground. (never to revive).
Owl’s Abattoir(Snowdon Drifts) – Challenge an Owl (I guess, as there is no challenger where it should be).
And in the north west corner of Ascalon Plains – There is a guy you challenge. (He too, wasn’t there), after an update he appeared and was challengable, but it is problematic I believe.