Without question, the most common mistake most coaches--new and old alike--make is Blaming The Freakin' Dice. I can pull any Let's Play video off the interwebs and, without even watching it, I can safely assume that someone, at some point in the video, can be seen/heard complaining about the dice rolls ... and how it's BECAUSE OF THE DICE--THE HORRIBLE HORRIBLE DICE--that they are losing, or that their players are getting pasted all over the pitch. It never fails. Of course, it's all B.S. The dice--in themselves--are nothing but a set of tools that provide a random result. It's the coach's understanding of team match ups, probabilities of various actions/positioning, and the notion of acceptable risk that determine success in a BB match--it's not the dice ... not on their own, anyway. If that were the case, then why even play the game? Why not just have each coach roll a die at the beginning, and whoever rolls higher--wins! Game over.
A case in point is a game from the TGS Tournament, between TotalBiscuit and Dodger:
https://www.youtube.com/watch?v=UyQB4kDMZzE
Now, any experienced Blood Bowl coach knows that a rookie Amazon team going up against Dwarfs is going to be rough. Granted, this is a Chaos Dwarf team, so there are a few hobgoblins who will be easier to deal with--but still--all that Block/Tackle ... it just negates the Amazon team's main starting perk, turning them from a psuedo-wannabe-dodgy-elfie-human team into a much slower, but just-as-crunchy, AG3-no skills, Skaven team. So, what does Dodger do? Why, she man's up and just marks every damn Chaos Dwarf player possible with her already-on-the-backfoot Amazons. Which, of course, doesn't end well for her in the long run. I mean, if you're just going to park your whole AV7, "totally countered" team right within arm's reach of the opposing coach's team, what do you really expect to happen?
Dice don't--on their own--determine the outcome of a match. Dodger clearly didn't understand the matchup--at all--and 'went HAM', not realizing TB had already countered her whole team before the game even started. Then she proceeded to play like an Orc coach, marking every player, every turn, as if to say, "Hey, please punch me in the face next turn, ok!!!" It's funny: TB actually let's it all slide until he starts really murdering her players, at which point, he starts to feel bad and begins to offer some suggestions (and condolences): "You might want to just stay away from my guys!" In the end, though, he pitch cleared the entire Amazon team. Watchagonnado, amirite?
And what got blamed? What did they talk about all game long?
Was is Dodger's unrelenting need to play "face check" with TB's Chaos Dwarf's? Was it the complete disrespect for the matchup? Was it the total disregard of what AV7 truly means for a team with no other skills (not really), against a team full of Block/Tackle/AV9? (And all the horrible probabilities that it brings; there is a huge difference between AV7 and AV9, in probabilistic terms.) Was it the complete ignorance and lack of elf-like, one-off, defensive play? (i.e., changing your gameplan away from bash, bash, bash.) Or was it the complete lack of targeting the hobgoblins (TB even goes so far as to say, "The hobgoblins are worse than Amazons in every way." It was like a giant neon sign, saying "Punch The Hobgoblins If You Want To Win!"
Of course not! None of these things got blamed, or were even talked about.
Almost from the first set of blocks the entire match was a gripe-fest about how good TB's dice were, and how--BECAUSE OF THE DICE--he was just 'murderizing' the Amazon team. If only TB didn't have such amazing dice!!!! Things would have been different!! 82% probability of failure be damned!! The DICE! IT WAS THE DAMNED DICE!!!
It's laughable and cringe-inducing all at once.
And, yes, it's the number one mistake most coaches make--old and new--partly because most people suffer from the Dunning-Kruger Effect, and partly because many people are unable to view the game from their opponent's point of view, i.e., they don't understand that their actions, player positioning, etc. all have impact in their opponent's turn as well (... and parking your players in positions that get them punched in the face on the very next turn is probably not a good idea.)
I view this kind of thing as a "mistake" because, quite frankly, you'll never get better as a BB coach if all you do is Blame The Freakin' Dice.