I was building with fences and I tried something, I jumped placed a fence and instead of jumping on it I went through it.
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It didn't Stop, I just tried it.
It will Stop is I move out of it
It's also doing it in pre 2
Dupe of MC-11242
Happens because fences are 1.5 blocks high in terms of the physics hitbox, but the player only jumps about 1.25 blocks high. This means that normally a player can't jump over a fence. However, any time a block is placed, the maximum space it checks to see if there is no player or mob in the way is inside the actual 1x1x1 block area, meaning you can place a fence in the block below you but your feet can still be inside that half-block-high hitbox above the actual block, and so, since it's all part of the one collision shape, you fall through.
Probably a clientside graphical error. Try restarting minecraft and see if it's gone. If it's still there, update this report.