Getting a bonus star for tallying a sack in a game situation where the 0-0 beginning-of-the-game tie leads to run-heavy CPU play calling that results in a bunch of field goal attempts after three-and-outs without a pass attempt.
Getting a bonus star for multiple tackles in a game situation where the CPU pursues an aggressive downfield aerial attack, where you can't allow much yardage, and where the CPU keeps chucking it deep to be caught (loss), incomplete (time's running out), or picked off (failing to earn the bonus star).
What challenge conditions have been most frustrating for you?