Missing an achievement

[Exbajalator]

On my achievements profile it says I've completed only 23 out of 24 of the achievements, but it doesn't say what the 24th achievement is.
Here's my profile - http://steamcommunity.com/profiles/76561197994407946/stats/bastion?tab=achievements

Bit annoying after completing them all, I just don't like it saying that it's still not 100% completed =/

[FallenAngel]

You're missing the "Turret Opera" achievement.

[Exbajalator]

Thankyou for replying! Don't know why they have hidden achievements, but anyway I'll go quickly grab it later on.

[bvguthrie]

The achievement is hidden so as not to reveal the Portal Easter egg before it appears in game.