Sneaking on a cauldron will make you fall inside it

Works as intended.
Sneaking only prevents you from falling a distance of 1 block or more. This is why you can fall off chests when sneaking on them, as they are not a full block tall. The same applies for the middle of cauldrons; the bottom of the inner part is not at the very bottom of the block, so there is less than 1 block distance to fall.

@Conor McS
That's still a bug. And the issue you talked about is MC-2334. Also, you don't fall outside the cauldron, but inside it.

Confirmed for 1.8.7; it also seems related to MC-2334.

Confirmed for 16w06a

I'm no longer able to reproduce it in 1.11. I'm able to reproduce in 1.10.2, it it must have been fixed somewhere in between.