Some blocks, that you normally build with, can't be used or can't be used normally with the cauldron. A few examples, ice blocks, mob skulls, etc. Some blocks can be used sometimes and not at other times. Sometimes you need to place a half slab over the cauldron then it will work. Like if you have Wool as an ingredient in your ingredient list and in a recipe you can sometimes get it to work and sometimes it won't. Occasionally, you will right click on the cauldron with the wool block and it will make the normal "hiss" noise and the block disappears from your inventory. But, most of the time it will make the "hiss" noise and simply place the block on top of the cauldron without removing it from the gameworld. Other times, it only places it on top and nothing happens, no "hiss" at all.

So, it'd really be nice to be able to use more types of items, like blocks and skulls, in the cauldron. These extra types of ingredients help to make more varied fantasy brews.

Fixed by this commit: 40d226a