What Blind Spots/Deckbuilding Mistakes Do You See Veteran Players Still Make?

Since this forum has a ton of enfranchised, veteran Commander players, I'm curious to know what blind spots you all see persisting in your playgroups, LGSs, and even in your own brewing. This is kind of the converse of new player mistakes; traps that still get you on your 100th list.

For instance, I've noticed that even in my more refined decks, I don't have lots of ways to check a player who has pulled far ahead early in a way that a removal spell or two can't solve. I'm looking for something more than "play more removal"--we're talking substantial shortfalls like failure to distribute mana production in the land base or folding to every graveyard-based strategy.

What's your cautionary tale for the EDH grinders, so we know what to look out for when we're deep into brewing a new list?