While most draftniks expect Zack Martin to be gone by the 20th pick of the first round or so, his "tweener" status (G/T) could cause him to drop. Players that drop tend to have a characteristic or two that scares teams--Star's heart issue, for example. If Morgan Moses continues to climb boards (some were worried about him not being there at #28 and even mentioned trading up for him), it could push Martin down. Throw in the fact that he has shortish arms, and it is possible he becomes the fifth tackle taken. 5 players at any position in round 1 is a stretch, so there is a strong chance that the 5th tackle in the draft is there at #28.
This is an article from the Observer a few months ago. Take a look at the video. Stranger things have happened. This was covered to some degree in February, but perhaps it needs to be revisited in light of developments (Cyrus, Tiny, Moses, etc) at T.