It is possible to put stacked items (e.g. swords or bottles) in the anvil and in the brewing stand.
Possible methods:

• shift click the item or
• put your stack into the quick bar, hold your mouse over the field you want to have the stack in and then press the number on which the item is

Stacking items is a feature that many multiplayer servers have and this bug (especially the second method) is very hard to fix.