One mistake that has burned me a few times and is fresh in my mind from last year: expecting my other league mates to make rational/predictable moves in the draft. For example, last year I really liked Jalen Hurts' value at QB. I was coming up in the 8th or 9th round and he was the top QB on my board by a long shot. I looked though and the 6 or 7 guys drafting after me all had taken a QB already. So I took another WR assuming those 6 or 7 guys that would all have two picks before me wouldn't be looking at QB. Then right before it came back to me, the person who drafted Dak two rounds earlier drafted Hurts. I jumped out of my chair was like "WTF are you doing, you just took Dak!". He didn't really have an answer, he just shrugged and said he didn't like the other players available at that spot. I kept just missing out on the QBs I wanted and it became a huge problem for me to start the season. I am pretty sure the guy I took instead of Hurts ended up being a total dud as well.
It happened to me a couple years ago as well when I drafted out of the 11 spot. The guy at 12 was an old school player that loves his RBs. I was positive he was going to start his draft RB-RB because he plays fantasy football like it's still 2004. So while I preferred 2 WRs to the top RB on the board, I figured I could take the RB in round 1 and when it came back around both WRs would still be available. Or at least 1 of the WRs, no way this guy starts his draft with 2 WRs. Well sure enough he went WR-WR and I got stuck having to make a pick that I didn't love.
So while it's smart to pay attention to the rosters and tendencies of your league mates, remember people are often unpredictable. People make weird decisions after a couple of beers when the pressure is on. Don't get too cute, take the players you like best or that your team needs.
This one is so critically important. The exact same thing happened to me last year when the team that took Hurts in the sixth then took Lamar Jackson in the seventh. Every other team had a quarterback already, so naturally, I took a wide receiver there.
I was absolutely dumbfounded
But you said it perfectly: you cannot expect people to behave rationally during a draft. When there is a ticking clock and they’re shuffling through papers, sometimes they will pick irrationally because of the pressure, and sometimes they will kick irrationally because they’re reaching for a player, and sometimes they are picking irrationally simply because they are not rational.
If your player is there and they are a value, scoop them up regardless of what the other teams have done. You just can’t predict with any degree of accuracy what’s going to be there for you in the next round.