Swift if statement not working. Clearly a bug, but whats the pattern

I have the following code fragment in an app. The 'if' statement wasn't properly evaluated. Even though score = -100 and bestScore = 200, the if statement if score < bestScore does not run the code within it. bestScore stays at 200.

If I add a print statement within the if, it suddenly starts working. I tried it in a playground without the print statement and it works fine. I've replace some expressions with 'true' for simplicity.

        var bestScore = 200
        var score = 200
        if true {
            if true {
                score = -100
            } 
        }
        if score < bestScore {
            print("Testing") // When this wasn't here it wasn't working
            bestScore = score
            bestMove = move
        }

I've seen this sort of strange behavior in Swift before, rarely, but I can't figure out a pattern to explain when this happens. This is with Xcode 6.1.1 (6A2008a).