Even some of the Hogwarts professors or students could catch Dumbledore. Maybe a curious student stumbles upon Dumbledore in a restricted area where he's doing something strange. Or a professor who has a different view on how to run the school catches Dumbledore in the middle of an action that they think is wrong for the school's environment.
There's also a chance that Dumbledore gets caught by a group of rogue magical creatures. For instance, if he was exploring an area that was supposed to be off - limits to wizards and the creatures there saw him as an intruder. They might not be as rational as wizards and could hold him captive without really understanding who he is.
In certain fanfictions, Dumbledore could get caught in a lie. Maybe he told the students one thing about a particular magical artifact, but then was caught when it was revealed he knew more than he let on. This would create a lot of drama as the students and other faculty members might lose trust in him for a while.
One possibility is that Dumbledore gets caught spying on a group of Death Eaters. This would put him in a very dangerous situation as the Death Eaters are extremely cruel and loyal to Voldemort. He might be tortured for information about the Order of the Phoenix. However, Dumbledore is very resourceful and would probably be able to hold his own against them for a while, giving his allies time to figure out a rescue plan.
In some fanfictions, Dumbledore is portrayed as having a lot of secrets. If he gets caught, it could be related to his past. Maybe some old enemies find out where he is and catch him. Then he would have to deal with his past coming back to haunt him. His friends might try to rescue him, but it would be a very difficult situation for him, and it could change the whole dynamic of the fanfiction story.
The story would take a different turn if Dumbledore gets caught. His capture could be the starting point of a new adventure for other characters. They might set out to prove his innocence or find out the real reasons behind his actions. This could involve a lot of detective - like work, exploring different parts of the wizarding world, and uncovering secrets that were long buried. It could also lead to a change in the relationships between the characters, as they might question their loyalty to Dumbledore or to those who caught him.
There may be such extremely non - canon and inappropriate fanfiction out there in the vastness of the internet, but it goes against the original nature and values of the Harry Potter series. J.K. Rowling's world has certain boundaries and this concept is far from what was intended in the original story.
Fred and George Weasley are very likely candidates. They are known for their love of pranks and their mischievous nature. They might see pranking Dumbledore as the ultimate challenge.
It could be Severus Snape. He has a complex relationship with Dumbledore. Maybe Snape discovers some of Dumbledore's plans that he doesn't agree with regarding Harry Potter. He then sends a Howler in a fit of anger to express his disapproval.
The Ministry of Magic could be responsible. They might be jealous of Dumbledore's power and popularity. They could see him as a threat to their own authority and come up with some false charges to arrest him. For example, they could claim he was hiding information about a dangerous dark wizard.
Well, a common theme is the re - evaluation of Dumbledore's actions. Since he was such a powerful and influential figure in the original 'Harry Potter' series, in 'dumbledore gets owned' fanfictions, his actions are often put under the microscope. Also, the theme of betrayal can be common. It could be that characters he trusted turn against him, showing that his trust in others was perhaps misplaced. This can lead to his downfall in these fan - created stories.