It is possible to use and emote without unlocking them. 

As someone who uses Keep Inventory a lot, I don't get achievements.
So when I was looking through the emotes and viewing achievements, it auto-added the emotes to my bar and allowed me to use them.
This means player who may have never gotten ANY achievements may use things like:
-  Diamonds to you!
-  The Hammer
-  Future unlock-able emotes.

This glitch will last until you reboot the world, causing it to turn back into that random emote you pressed before.

How to redo this glitch:
Step 1: Boot up the world and press B.
Step 2: Click "Change Emotes"
Step 3: Click on one of the emote slots (I chose the last one) and click on one you don't own.
Step 4: Click on a random one, then spam the one you don't own.
Step 5: Now, click the "View Achievements" button.
Step 6: Finally, exit the menu and press B.