If you've played any of the past Zombies maps from previous Treyarch-developed Call of Duty games, you're doubtless aware that the studio likes to include deviously tricky easter eggs for players to unlock.
These easter eggs put all other secret achievements to bed with their complexity and level of challenge. Gone are the days of simply taking a hidden path and unlocking the easter egg or putting in the Konami code at a certain place and time. Treyarch has raised the bar time and time again, only to have it raised further...by themselves.
In Mob of the Dead, Treyarch may have outdone themselves.
The easter egg in Mob of the Dead - the Zombies map in Black Ops 2: Uprising - gives you the achievement 'Pop Goes the Weasel' and this is probably where you want to stop reading if you're hoping to avoid spoilers.