This happened to me yesterday and it needs to be corrected. If a gameplan change doesn't work with the game logic, the game needs to prompt you that there's an issue, rather than allowing you to keep going and games not sim. I made ZERO changes to depth chart or playbook between the game that simmed Monday and yesterday's 1st half, yet NONE of my formations would work yesterday, my second half wasn't simmed and I couldn't run test games. I went through my entire depth chart and each offensive and defensive playbook with a fine tooth comb, looking for 0's and changed any 0 to a 1... still nothing. I ended up building one new O and D playbook last night from scratch and they seem to work. I WILL NOT DO THIS AGAIN. If this game stops simming again, I'll walk and they can keep it. I'm not going to invest that much time into it when the engine itself should tell you where there's a problem and quite frankly, now it's no fun to even sim games, knowing that if I make a slight change, the damn thing may stop simming and then I'm back to trying to figure out what the issue is. Hell, I can't get anything to work in the passing game or running game anyway, aside from running my QB every play and even that playbook, which has worked fine for hundreds of simmed games, stopped working yesterday with NO tweeks to cause it.
And yes, I realize it's BETA and there are bugs to work out, but it kinda defeats the purpose of beta testing when you're afraid to try different things for fear the game will stop working.