This dream must have taken place at the beginning of Frodo's journey. He met a gypsy fortuneteller who offered to tell him his future. When he refused, she offered to turn him into Aragorn instead, and send him forward in time. Frodo accepted, and after he was transformed, the gypsy sent him forward in time to Amon Hen.
It was just after Boromir had tried to take the Ring from Frodo, and Boromir was sobbing on the ground. Well, Frodo—now Aragorn—immediately went over to Boromir and raped him, like in those awful fanfics I've read. It was a horrifying scene, and afterwards, Frodo-Aragorn was horrified by what he'd done. He decided to go back in time and get the fortuneteller to change him back to his normal form.
As he traveled back in time, all the normal scenes from Lord of the Rings were changed, and the new scenes belonged in awful, cliched fanfics. For example, Aragorn and Arwen were having sex in the garden of Rivendell, and Legolas was teaching a Mary Sue how to shoot arrows. Apparently, by being turned into Aragorn, Frodo had changed history so that it resembled bad fanfiction. As he went back in time, he got angrier and angrier, and resolved to make things normal again.
Finally, he met the gypsy fortuneteller and demanded that he be changed back to his normal form, as he was so unhappy with the way events had occurred. The fortuneteller said something like, "Well, what did you expect? If you change one aspect of history, you change everything else about history too."
This made Frodo even angrier, and he shouted at her, in a strange hick accent, "Well, if ya don't change me back an' fix stuff right now, I'll whale the hide off ya!"
Whether this threat worked or not I never knew, because the dream ended there.