Q: Can dispensers (with a bucket) empty a cauldron? I've been experimenting in 1.17.1 (Java), particularly with the new dripstone. However, I've been unable to figure out a way to empty filled cauldrons. At first, I thought it was a problem of aiming the dispenser correctly, but I've now got a setup where it is directly above the cauldron and aiming downwards.
Can I expect this to work? Or will it just dump the empty bucket into the lava cauldron?

A: No, that is not going to work. A dispenser cannot perform that type of functionality. This also applies to attempting to dispense into cauldrons, it will just dump a full bucket out.
Strangely enough, this is actually a bug: when items that would behave specially are inserted into the dispenser but the block conditions aren't met, the dispenser should just stop instead of ejecting the item like a dropper, similar to how bonemeal works, not ejecting the bonemeal if there is a problem.
