Placing a trapdoor next to a fence (or fences, if stacked vertically) then standing between them and opening the trapdoor allows you to phase through the fence(s).
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Misleading title. Entities still collide with the fence. You just found a way to change the collision box of the fence while standing within its new bounds.
Some other ways to get this are:
extending a piston towards the fence post
pushing blocks next to the fence post
placing a block while jumping(you land on to of the placed block, but since a fence is 1.5 high, you can still move inside)
it also affects glass panes, iron bars and even walls if the walls also connect on the opposite side(up: false on F3 screen)
a similar behavior is when standing inside a door and opening/closing it
Can confirm.