There are exceptions to every rule.
Tony Mandarich, the Green Bay offensive tackle you mention, was not only the highest rated tackle in that draft, he was the highest rated PLAYER in combine history up that point according to BLESTO which was a popular scouting service back then.
Turns out Mandarich was a steroid monster of the first order and once he couldn't take the steroids anymore he was nothing.
In fact, the Packers are probably not a good example of getting good value in the 1st round for offensive tackles -- last year their 2010 1st round RT (Brian Bulaga) and their 2011 1st round LT (Derek Sherrod) started a grand total of ZERO games due to injuries.
In 2012 they started only 7 combined games.
However, players at any position can get hurt.