If you spawn in a mob egg in creative mode, then switch to adventure mode you can not use the mob egg.
MC-77689
Can't use mob eggs in adventure mode
Resolved
Duplicate
0
SupahYayaz
(Unassigned)
Unconfirmed
Minecraft 1.8, Minecraft 1.8.1, Minecraft 1.8.2-pre4, Minecraft 1.8.2-pre5, Minecraft 1.8.2-pre6, Minecraft 1.8.2-pre7
Retrieved
Thank you for your report.
However, your ticket has been resolved as a duplicate.
This issue has been linked to the original bugreport: MC-65170. You can find or add further information on that report.
Please search before reporting any bugs, as it's likely that one exists already.