Here's another one that bothers me that I'm starting to see.
The best player at his position is one of the best players in the draft
Obviously we know this isn't true because it clearly doesn't apply at many positions (P, K, and LS come to mind) but it doesn't stop people from assuming that it's true at other positions. Every year you're going to have a "best" guy at a given position. If you rank prospects, you've got to have one guy who comes before the rest. That doesn't mean he's any good, though and we should avoid the hagiography of a guy because he's perceived as better than everybody else at his position. After all, the perceptions of any analyst (and indeed scout) need not have all that close a relationship with reality.
The example of this that really stands out to me is the 2006 WR class. The consensus among many media outlets was that Chad Jackson was the best WR in the class, so talking heads at ESPN kept insinuating that he was a first round pick, and indeed a high first round pick (I believe in the eve-of-the-draft live mock on ESPN they had him in the top 10.) Guy went in the second round, couldn't play at all, and was out of the league within four years.
We saw a small version of this last year with the love for David DeCastro. Now unquestionably DeCastro was a really good guard prospect, but he's still a guard. There were folks in the draft-analysis media who kept insisting that he was a top ten pick and a player of rare value, and he ends up going 7 spots lower than Iupati did two years prior (the jury is still out on whether it's a good pick, admittedly.)
So I would caution people not to fall in love with a guy just because he's the tops at his position. You may need to compare him to players who play other positions (and the relative values of those positions) to get a better handle on the draft.