In 1.7.10, you could summon a Wither Skull by typing "/summon WitherSkull". In 1.8 pre 1 however, it says 'Unable to summon object" despite the WitherSkull name still appearing in suggestions after pressing Tab key.
MC-69004
Cannot summon WitherSkull
Resolved
Duplicate
1
John Smith
(Unassigned)
Unconfirmed
Minecraft 1.8-pre1
Retrieved
Also happens when trying to summon a Fireball
Did not happen in 14w34a (unsure about b,c or d)
dupe of https://bugs.mojang.com/browse/MC-68740