When Swift arrays aren't Swift arrays

A quick read of the Swift documentation tells us that Swift arrays must contain objects of the same type, so this would generate an error:

let myArray = ["String 1",1]

But something that caught me out was that when you import the Cocoa framework, Swift uses NSArray instead, which is full of AnyObject, and the above will compile and run just fine. What makes this a real gotcha (or at least, did for me) is that by default playgrounds import Cocoa at the top. 

Ray Wenderlich's Swift syntax cheat sheet→

A useful primer!