Things out of my control. Spending days tracking down a bug only to find that there is no mitigation other than modifying some upstream library. Bonus terror:

Things out of my control. Spending days tracking down a bug only to find that there is no mitigation other than modifying some upstream library. Bonus terror: deploying the changes would involve numerous clients also out of the team's control. Cue weeks and months of begging to make the upgrades. (Edit: one more:) Deploying changes that don't have an easy rollback mechanism. E.g. a risky change involving apps or browser cookies, and both deploy and rollback take e.g. a day.