The mob imprisonment tool when crafted, doesn't have any NBT (not even a root tag). But when you've used it once and empty it again, it will have just an empty root NBT tag. This makes it different from the freshly crafted one, and this means they cannot stack with each other in the infinity backpack.

This is annoying when you prepare on a journey and get a ton of new tools on the go, put them into the backpack and when you use one of them to move a mob from here to there, it then suddenly doesn't stack with the rest anymore if you want to put it back. Sounds like a reasonable use-case to me.

How to reproduce:

• Craft two new mob imprisonment tools (or use creative inventory)
• Spawn a mob
• Pick it up with one of the tools, and immediately let it go again
• Go into creative and get an infinity backpack
• Put one of the mob imprisonment tools into a slot of the backpack
• Try to shift-click the other tool into the backpack too (notice it doesn't go in)
• You can even switch them around, same result.

Industrial Foregoing Version: 3.3.1.5-9

Titanium Version: 3.5.6-38

Crashlog If Applicable (Upload to pastebin/gist):