If you are a fan of the Vikings, perhaps the single biggest example of EXHIBIT A for ignoring the preseason should have been Dante Culpepper in 2005. I bring this up every few years, so what's once more?
In 2004, Culpepper was the #1 fantasy QB, which is insane given that Peyton Manning threw for 49 TD that year! The Vikings were loaded offensively. They rolled out Randy Moss, Nate Burleson, and Marcus Robinson, all three fantasy start worthy in the day, along with the #8 fantasy TE in Jermaine Wiggins.
The Vikings offense in the 2005 preseason looked absolutely, positively unstoppable. Culpepper played the first quarter of each game and put up a 36-44-520 total passing line. That's an 82% completion rate with an 11.8 YPA. His stock went through the roof, and at the time he was going Top 5 OVERALL in most of my drafts. In a couple of leagues he was the first pick. The hype was insane.
Then the regular season came and he had 8 INT's in the first two games. He had 12 INT's and 7 fumbles in the first 6 games. He got knocked out early in his next game with the trifecta of a torn ACL, PCL, and MCL and never played for the Vikings again.
I chuckled, as I didn't own him in any of my leagues (and at the time I played in a TON of leagues). But that should have been the magic moment for people to tap the brakes on how things look in the preseason and to avoid greatly changing your opinions based on players facing vanilla defenses, potentially playing against players that would be working at McDonald's in a few weeks, or otherwise going up against guys only giving about 60% effort.