He could get seriously injured. In some fanfictions, he might break a bone or two. It would also likely lead to him being out of the Quidditch match if it happened during one.
One consequence could be that he loses confidence in his flying abilities. After falling off his broom, he might be hesitant to get back on it right away. In some fanfictions, this event could be used as a way to develop his character, showing his vulnerability. His enemies might also take advantage of this situation. If Voldemort's followers saw him fall, they could try to attack him while he's in a weakened state.
In some fanfictions, his fall off the broom could be a turning point in the story. It might lead to him discovering a new magical ability. For instance, he could unconsciously use a form of magic to slow his fall or even levitate himself slightly. This newfound ability could then be explored further in the story, perhaps making him a more powerful wizard in unexpected ways.
Dumbledore could also be a savior. Given his powerful magic, he could easily appear at the scene and use a spell to stop Harry from hitting the ground too hard. Or perhaps one of the teachers at Hogwarts, like Professor McGonagall, who is strict but caring, would use her transfiguration skills to create something to break his fall.
He might get confused. Fanfiction often takes the original story in different directions, with different characterizations and plot twists. If Harry reads it, he might start to question the 'true' nature of his own experiences and the people around him.
One consequence could be that he endangers himself. Maybe he disobeys a warning not to enter a certain dark forest in the fanfiction. As a result, he could encounter dangerous magical creatures or dark wizards. Another outcome is that his disobedience might lead to a rift in his relationships. If he doesn't follow the advice of his mentors like Dumbledore in the fan - made story, it can cause tension between them.
When a kid falls 12 stories, the outcome is almost always tragic. The acceleration due to gravity will cause the kid to gain a high velocity by the time they reach the ground. This high - speed impact can break bones all over the body. The internal organs are not built to withstand such a violent impact, so internal bleeding is a common consequence. If the kid is lucky enough to survive the initial impact, they will still face a long and difficult road to recovery with many surgeries and rehabilitation procedures likely.
She could suffer very serious injuries or even die. Falling from such a height means hitting the ground with a huge impact force, which can cause broken bones, internal bleeding and head trauma.
The girl might break multiple bones in her body, like her legs, arms, and ribs. There could also be internal injuries to her organs. Head trauma is very likely too, which could lead to brain damage or concussion. If she lands on a hard surface, the impact would be much worse compared to a softer one.
In certain fanfictions, Harry's fall could be a part of a larger conspiracy. Maybe someone tampered with his broom on purpose. This would then lead to an investigation, with Harry and his friends trying to figure out who was behind it. Along the way, they could uncover other secrets about Hogwarts or the wizarding world in general, which would add more depth and complexity to the plot.
One consequence could be that his fans in the fanfiction community will be disappointed. They might have been looking forward to his new works.
One consequence could be that Harry becomes more isolated. If he hates the Order, he may distance himself from them, which means he loses a group of potential allies. This could put him in a more vulnerable position in the wizarding world.