Hidden Trail (AKA Brood Mother Cave) shortcuts [Obsolete?]
Requires: Having health before the final plunge due to fall damage
Vid:

Just hold to the left and air attack as you pass the platforms. Prevents death from fall damage, avoids getting elevator lever

Shortcut to Brood Mother fight
Requires: Fledgling's Blood Soul, Item Mastery skill (General tree, middle branch, 2nd option)
Vid:

Use an air attack to kill your air momentum and climb up the left slope/ledge, then cast Fledgling's Blood Soul mid-air and die. The checkpoint will respawn you mid-air and you'll land in the boss fight with full health.

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Sliding Out Of Bounds
Requires: Sliding
Vid: https://www.twitch.tv/koatal/clip/AttractiveOutstandingKaleBibleThump

When sliding through a passage that requires a slide, go to the equipment menu and change weapon. Forces character to leave slide animation and clip OoB.

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Village-Valley Wrong Warp
Requires: ???
Vid:

While climbing the ladder and transitioning from the Village screen to the Valley screen you can be spawned much higher on the map, near the exit to Secret Trail. Unsure of exact requirements.

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Mine platform skips
Requires: Any jump height increaser (Recurve Bow, Time Deviation, etc.)
Vid:

Cuts out the need to wait around for platforms as much. Might be obsolete in the October patch if you can reach EFC boss via the Limestone Way route.

Great work with the videos, thank you.

The wrong-warp happened to me once, i wasn´t able to recreate it since.
Seems like it´s pure mashing jump while holding up?

That's the one of the common occurrences the both times I've gotten it. I've also never gotten it first try, so maybe you need to go into Valley (to load the map and such) and go back into Village, before trying again. Maybe falling into Village (opposed to going down the ladder) is necessary. Hard to say until there's more successes (w/ inputs)

Here's the other successful time.

You can consistently skip the whole Valley by warping out of bounds from the ladder:

You can also use time dilation to skip a whole section of the mines: [21:39] as well as part of the flooded zone [29:09]

I just realized how huge the slide out of bounds is. Off the top of my head, i remembered 3 spots that can be used (and i tested them - they work). I've marked them in red on this map: https://imgur.com/reD50bL

Using slide oob on the spot that is in that twitch clip and then walking left allows you to enter load zone that enters upper underground hall

The OoB sliding thing has a lot of useful locations. But it also allows you to skip right into the Underground Halls at the end of the game rendering them a lot of obsolete to the MASSIVE one.

@BabyPigmanBabyPigman What inputs are you doing when jumping into the Valley map?

I just keep jumping with the stick in the top right position, then while it's loading the next area i move the stick completely right and mash jump
I haven't experimented if there's another way to do it, but it's easy and consistent

Wasted House Sliding OoB example:
Vid:

You'll wanted to go into the Equipment menu when you're about to leave the screen (extreme left) so that you won't fall inbounds. You can bring up the menu overlay and highlight Equipment before starting your slide.

If you get caught inbounds you can reset yourself by going 2 screens to the right.

Keep left once you've gone into the Wasted Warehouse screen to avoid falling into a cavern with a chest, this'll require you to to the glitch again.