This is a minor bug since 0.16.0. When you tap a spawn egg on a monster spawner, it should change the spawner into whatever mob you chose, but since 0.16 it also consumes that egg and spawns a mob.

What should happen:
Tapping a monster spawner with a spawn egg should change the mob inside the spawner.

What Now Happens:
It also uses that egg to spawn one mob beside the spawner.

Steps to Reproduce:

• Place a monster spawner
• Tap it with a spawn egg
• Notice that a mob gets spawned with the egg