I was fiddling with spawning structures with spawn eggs on a redstone test server one day, using a system that was originally built in 1.8. When the structures failed to spawn, I tried selecting the entities by hand, and got "That entity cannot be found" errors.

What should happen:
1.) Spawn Chicken with spawn egg
2.) Run command using @e[type=Chicken] selector
3.) Command selects chicken

Instead:
1.) Spawn Chicken with spawn egg
2.) Run command
3.) Get "That entity cannot be found"

Reproduction:
1.) Spawn a mob with a spawn egg
2.) Attempt to select it

It's a cool mechanic to be able to do this, and I'd like to see it back in action.