Causes an error in the playground, which is expected. What's unexpected is the line on which the error occurs.

Forced unwrapping causes errors elsewhere.swift

// Playground - noun: a place where people can play
 
import Cocoa
 
var str:String? = "Hello, playground"
 
str = nil
 
if let actualStr = str {
    println(str)
} else {
    println("found nil")
}
 
let forcedStr = str!