As reported on the Revelations forum: HERE
Also reported to Tinkers Construct GitHub HERE

FTB Revelations 1.5.0
Tinkers Construct 2.9.0.55
YABBA 1.0.11

1. Create Yabba barrel. Go into settings (shift right click) and make sure it's locked and always display data)
2. Place any stackable item in the barrel.
3. Place Tinkers Construct crafting table next to barrel.
4. Go into crafting table, pull out items from barrel then place them back in. (The trick is putting them back in)
5. Exit crafting table.
6. Barrel contents should read as 0.
7. Left click barrel like you are putting items in and amount of item will be doubled. This will happen even if contents of barrel are full and leave invisible dropped stacks on the ground.