Overview: Using this bug, you are able to put unstackable items into stacks.
Items that are affected (so far)
Boats, Soup, Music discs, totems of undying, the various buckets, minecarts, potions, shucker boxes and saddles
Steps to recreate:
1. Make a new creative world
2. Take out 2 curse of vanishing books and 2 of the same item. ie totem of undying
3. Using an anvil apply a vanishing book to each totem
4. Now go to a grindstone and place a totem in each slot, the output will be 2 stacked totems, you have to shift click them to your inventory.

To stack even more, you would repeat steps 2-5 until you have as many "stacks" of 2 as you want. Then you click on one "stack" and just click on the other "stacks" and that adds more items to it.

Notes:
Using this with enchanted books and an anvil causes things to get weird. (further testing needed)
In survival I believe you cannot put the vanishing books on an item, but maybe villagers could sell an item with the curse (more testing needed)

I am able to make a video if needed