I'm not naming any draft picks because there is so much chance involved with picks. Pretty much all the experts agreed that Richard Sherman looked like a really stupid pick at the time it was made, for example. Sometimes "dumb" picks look smart and "smart" picks look dumb.
1. Charlie Whitehurst. This move was dumb before the fact, dumb after the fact. The kicker is that if Seattle hadn't made this mistake, Arizona would have.
2. Trading Rob Sims for a 5th rounder. Sims was an above average LG who ironically would have fit Tom Cable quite nicely, unfortunately Alex Gibbs didn't like him so off Sims went. He's remained a quality starter for Detroit each of the last 4 NFL seasons, while LG has been a pricey turnstile in Seattle. Thankfully, we got Kam Chancellor with that 5th round pick. Maybe we still get him without that pick, maybe we don't. But random draft luck aside, dealing a quality left guard in his prime for a 5th was stupid.
3. The 2013 roster cutdown decisions. Jaye Howard, Allen Bradford, and Michael Brooks flashed ability in the 2013 preseason, but they were let go for bums like Mike Person and John Lotulelei. Person and Lotulelei didn't last long, thank goodness, and we did manage to keep the best player of the bunch (Brooks) on the PS, luckily. Bradford I kind of understand because he was out of cheap team control years, but Howard still has two years left on his deal, and he'd be a great candidate to replace McDonald right now. Unfortunately, the Chiefs grabbed Howard and haven't let go.
4. Matt Flynn. I hate calling this a mistake because $10 million in guaranteed money seems like a reasonable price to pay for a franchise QB gamble. We also got a 5th rounder out of it, and though I doubt it matters a ton, Flynn contributed to the QB competition that helped forge Russell Wilson's legend. The addition of Flynn also helped act as a smokescreen for Seattle in that 3rd round. A few teams might have moved up ahead of us in that frame for Wilson, but didn't because they had seen that we just paid big bucks to Flynn.
5. The Bruce Irvin thought process. Drafting Irvin in and of itself wasn't a mistake. The mistake was drafting a super-raw pass rusher and then banking on him in year one to contribute with no other backup plan in case Clemons got hurt. Irvin's rookie season wasn't good nor was it awful, it was exactly what a realistic person should have expected, but when we needed Irvin to be like Clemons in Atlanta, he couldn't do it. Because he's not that guy. Since then Seattle has overloaded on proven veteran pass rushers. It won them a Super Bowl.
6. Letting Breno walk for what was essentially a 2/7 contract (albeit fully guaranteed). Maybe we get lucky and Bowie is ready next season, but he wasn't ready in 2013. Breno had his best season yet in 2013. If Bowie continues to get Wilson killed this will look like a huge mistake.
7. Keeping Turbin at KR for as long as they did.
I'm sure there are many more mistakes, but I can't remember them. Probably because PC/JS are so good about learning from their mistakes quickly and fixing problems properly without letting ego get in the way.