1) Not capitalizing on turnovers (especially at home). You know this scenario, your team gets an early turnover deep in opposing territory (say the red zone or just outside). They go for the quick play to the endzone and miss. They hand it off for maybe 2 yards. Third down they get maybe 3 more. 4th and 5 less than five minutes into the game and the coach goes for the touchdown. It fails. This is terrible on every front. Their defense knows your team can be stopped. Your offense fails to score points. All positive momentum is stymied on four plays, at home this can suck the life out the fans making it easier for the opponent to really seize momentum. The better plan is to go for it all, but accept 3 points when they are there. Of course you want touchdowns, but it makes the opposition feel worse when they turn the ball over AND give up points less than a minute later.
2) Being down by multiple scores and mismanaging the clock. Team down by 4-11 points and 6 minutes or left remaining. The team gets to the red zone, but stagnates. Yes we all want them to push on and make it happen, if this were a video game we would save the game and reset the scene until we succeeded. Our team doesn't get that luxury. March in, go for the quick score. Go for the two or three plays that either work or end in a penalty on the other team to get that touchdown. But know that it is a two score game and running into a brick wall defense is only going to kill the clock. Take the first score presented to you and allow your fans to hope for the best.
USC v Notre Dame was a prime example of this going wrong. Around 5 minutes remained in the game and they were within the red zone. They tried the same fade route to Marquis Lee on back to back plays. Both were under thrown, both resulted in pass interference penalties. Lane
Truly these are the two "killer" scenarios that came to mind for me. Icing the kicker might be one, but given how terrible kickers have been this year overall I decided to give them a pass. Kickers can't win and their coaches shouldn't organize their play selection to make them out to be the goat. I know there are others and I am curious to know what irks you most as a fan. Let me know in the comments below.