Right now on Povar, there are 2 untargetable giants flying about the obelisks in the giant area. They have been there for a while. It reminds me of back in SoD, when the giants near the catapults in Koarscian Warrens would fly and you couldn't get them down.
I've seen this before on Xegony, but have always managed to get them down by /tar them (sometimes have to run from a hill ... maybe one time I used rocket boots) and sending a pet in. Even that not working? Might try a /petition to reset the zone ... would probably be faster going into the weekend.
Theres a large collision box around the crystal structures and other geometry in the zone where you won't even be close to touching the texture but you'll collide with the wall. You can run up most of them, and so can mobs, where the pathing gets confused and can't get them back down. You cannot click through the collision box so it may be the invisible wall that prevents free targetting, but Crag said it /tar will work and pets can aggro stuff thats stuck behind things usually. WW just has lots of puliing/pathing warp issues.