Todos that are on your list but rely on someone else to do. When something is your responsibility or affects you or you're dependent on it for some later work, but you can't just do it yourself. Even if the other person is totally cool and reasonable it's stressful because you have to be like "eh, is this done yet?" and you feel bad about pestering them too much, etc.
Things that you know need to be done but aren't your responsibility or you can't make someone do it. For example when you're a coder and you're working on a game and you think the art or design or sound or whatever is really doing something wrong, and maybe you try to say something about it nicely but nobody pays attention to you, and day after day you just keep thinking "ugh this sucks it needs to be different" but you can't do anything about it. Or in code when you're a low level guy and you think the engine architecture or coding methods or something basic are totally fucked but the lead doesn't want to do anything about it.
Expectations that are not voiced or even opposite-voiced. One of the great stress causers is when your boss/lover doesn't tell you what they need from you, and then gets mad at you for not doing it. You wind up being like "ahh, I didn't know, what did I do?". Often expectations are even opposite-voiced, as in when your boss says "I don't expect you to work overtime or come in weekends" , but when somebody does do those things they get a bonus and the boss loves them, or when your lover says "I don't need you to buy me jewelry and flowers all the time" when in reality they do. Of course the boss/lover is not entirely to blame in these scenarios, a certain amount of opposite-speak is to be expected and it is the mature man who can ignore the literal words and listen to the actual meaning. However, when you are put in the situation that you have to guess what you're supposed to do, the penalty or reward can seem capricious and you wind up constantly in fear that you aren't doing something that you should be doing, which leads to extreme stress.
Tasks that are unclear, especially when you don't have the freedom to really do them the way you want, or you're told that you have more freedom than you really have. The worst thing is probably when the task-giver actually does have a clear idea of what they want, but doesn't tell you. They'll just tell you something vague like "optimize the game", so you go off and do some things, then come back and they go "oh no no, not that, more like this". Usually asking doesn't help, you'll be like "do you want this or this?" and they say "oh whatever, I don't care", but then you do something and they go "oh, not that".
Conflicts of reality / expectations that you know are false. This is very common in games where the producers and leads are telling everyone that you're going to ship in 6 months, but you've looked at the game and you know damn well it's more like 12 months. There's a conflict between what you know to be the reality and what they are claiming, and this causes constant stress, because they are giving your crunch-like tasks to wrap things up and you know they aren't the right tasks and the crunch is unwarranted, etc.
As an underling who is afflicted by these stressors, there's not really a whole lot that you can do about it. But as a stress causer, you can be aware of what you are doing to others and try to reduce it.