I'll share one mistake i witnessed yesterday. Unlike most mistakes in this thread, its not a stupid, obvious mistake, but its a situation that can instructional and serves as a lesson:
My opponent was Viy. I was playing dwarves, im not a fan but i know how to play em.
I have Yrden in this deck (like many dwarf players do), so my goal is to win R1 without using it, so i commit a lot of pointslam.
My opponent takes too long to set up his thrive and consumes for Viy. I was holding back my Brouver until this point, as soon as i saw Viy i played Brouver, because we all know Viy rarely pack any form of removal.
I was quite ahead, so i immediately passed. The mistake of my opponent was he was only looking at the current points, completely ignored the huge boosts Brouver was getting me (8pts per turn). He eventually gives up, so i win R1 AND have card advantage.
Then i went for the 2-0, although this was a mistake - it succeeded, i did the 2-0, but i had even higher chances of winning if i didnt push and went to a longer R3 with 2 cards up and Yrden and artefact compression in hand.
Anyway, the main lesson here is that one of the main aspects that separates a bad player from a decent one is the ability to plan ahead, dont just look at the current state of the round, take consideration of the passive effects, and dont play mindlessly - if you're on R1 and you're getting close to the essential 4-card mark, and you're too far behind, you risk going a card down even if you're on red coin, if your opponent passes.
Another example i did yesterday, same deck, but vs Arachas - we were at 5 cards, i was quite ahead, i did the math, the biggest play he could possible make in 1 card is a Triss TK+bone talisman, if he used leader to fill board that plays for a max of 22 pts, that wasnt enough, so i passed. He obviously didnt do the math, tried (but with bone talisman, no triss TK), failed and forfeited.