Embarrass your protagonist. Make them seem weak and vulnerable in some way.
Shoot someone. That always takes the reader by surprise.
In relation, kidnap someone. Or, rather, make it seem to your protagonist like someone has been kidnapped.
Have one of your side characters disappear or become unavailable for some reason. This will frustrate your protagonist.
Have someone kiss the wrong girl, boy, or person, especially if you’ve been setting up a romance angle. It’s annoying.
If this story involves parents, have them argue. Push the threat of divorce, even if you know it won’t ever happen. It’ll make your readers nervous.
Have someone frame your protagonist for a crime they didn’t commit. This could range from a dispute to a minor crime to a full-blown felony.
If this is a fantasy story involving magic or witchcraft, create a terrible accident that’s a direct result of their spell-casting.
Injure your protagonist in some way, or push them into a treacherous scenario where they might not make it out alive.
Have two side characters who are both close to the protagonist get into a literal fist-fight. This creates tension for the reader, especially if these characters are well-developed, because they won’t know who to root for.
Make your protagonist get lost somewhere (at night in the middle of town, in the woods, in someone else’s house, etc.)
Involve a murder. It can be as in-depth and as important as you want it to be.
Introduce a new character that seems to prey on your protagonist’s flaws and bring them out to light.
If it’s in-character, have one of your characters get drunk or take drugs. Show the fallout of that decision through your protagonist.
Spread a rumor about your protagonist.
If your protagonist is in high-school, create drama in the school atmosphere. A death of a student, even if your protagonist didn’t know them personally, changes the vibe.
If your story involves children, have one of them do something dangerous (touch a hot stove, run out into the road, etc.) and show how the protagonist responds to this, even if the child isn’t related to them.
In a fantasy story, toss out the idea of a rebellion or war between clans or villages (or whatever units you are working with).
Add a scenario where your protagonist has to make a choice. We all have watched movies where we have screamed don’t go in there! at the top of our lungs at the main character. Make them go in there.
Have your protagonist find something, even if they don’t understand the importance of it yet. A key, a document, an old stuffed animal, etc.
Have your protagonist get involved in some sort of verbal altercation with someone else, even if they weren’t the one who started it.
Let your protagonist get sick. No, but really, this happens in real life all the time and it’s rarely ever talked about in literature, unless it’s at its extremes. It could range from a common cold to pneumonia. Maybe they end up in the hospital because of it. Maybe they are unable to do that one thing (whatever that may be) because of it.
Have someone unexpected knock on your protagonist’s door.
Introduce a character that takes immediate interest in your protagonist’s past, which might trigger a flashback.
Have your protagonist try to hide something from someone else and fail.
Formulate some sort of argument or dispute between your protagonist and their love interest to push them apart.
Have your protagonist lose something of great value in their house and show their struggle to find it. This will frustrate the reader just as much as the protagonist.
Create a situation where your protagonist needs to sneak out in the middle of the night for some reason.
Prevent your character from getting home or to an important destination in some way (a car accident, a bad storm, flat tire, running out of gas, etc.)