We'd put up a spoiler warning but the title kinda gives that away... and the fact that at some point in the game you'll be able to temporarily visit a different location isn't too big of a spoiler.
In Chapter 5 of Red Dead Redemption 2, players will wind up on the Caribbean island of Guarma. After the story takes you back to the main map, going back to the island is impossible - or at least, should be.
There is no proper way of getting back to the island, but just like how the will of developers was unable to keep players out of Mexico, someone found a way to get to Guarma too. The island is home to a couple of animals not found on the main map, including one which can only be found if you use this glitch because it is located in a part of Guarma not accessible traditionally.
You'll need to be extremely patient to complete this glitch, however. It requires a lot of rowing with a canoe, much of which is done under the map. What you need to do is go to the Lannahechee River on the east bank, right across from Annesburg.
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You then need to keep rowing northeast while hugging the eastern bank of the river till you go beyond the bounds of the map. Eventually, your surroundings will degrade in terms of graphical quality and prop density, and you'll reach a plain, grassy lick of land.
You need to row around this and proceed into the shore. We say "into" because this low-poly mesh has no collision detection and you'll just row through it finding yourself below the map texture.
Since the way Rockstar did the game map, Guarma is in the same physical instance as the main world, just situated under it. By glitching under the main map Guarma will load up. You'll once again need to row for an excruciatingly long time to pass under just about the entire main map before reaching land in Guarma, where you'll finally be level with the ground texture again. Now that you've completely worn down your X button, you've reached your destination.
At this point you can just waddle ashore. It's recommended to attempt this glitch with a Red Dead Online character or before Chapter 6, as once Chapter 6 triggers the player controlled character will have a much harder time should the canoe capsize. If you manage to say under the map or on the shoreline while in Guarma, you can reach the out of bounds lower reaches where a lone beached tiger shark corpse will be found. You can't interact with it, but it's a sign of a species that was considered during development but ultimately cut from the final game.
There is also a method of reaching Guarma with a horse, but that requires a lot of consumables and just isn't as reliable. Props to Basement Gamer Bros for posting this particular glitch on YouTube.