Command blocks cannot /summon any entity.
What I expected to happen was...:
It would summon the specified object.
What actually happened was...:
Nothing happened and the command block output said 'Unable to summon object.'
Steps to Reproduce:
1. Get a command block.
2. Input a '/summon' command using any entity.
3. Activate it using redstone.
Entity ids changed in 1.11, make sure you use those.
Any other case: cannot reproduce.