You wouldn't expect calling a method on a class to modify the variable holding a reference to the instance, but it can happen.

Mutable Class Method.swift

//: Playground - noun: a place where people can play

import UIKit

protocol MagicProtocol {
}
extension MagicProtocol {
    mutating func magic(xyzzy: Self) {
        // Mutating methods defined in a protocol extension can mutate `self` even if
        // the protocol is implemented by a class. 
        self = xyzzy
    }
}

class MagicClass: MagicProtocol {
    let value: Int
    init(_ value: Int) { self.value = value }
}

func test() -> Bool {
    var a = MagicClass(42)
    let b = MagicClass(23)

    a.magic(b)

    return a === b
}

test()

This is very annoying when you're trying to create a proxy wrapping an array — a lot of everyday methods like insert and removeAtIndex are defined as mutable extension methods on a protocol like RangeReplaceableCollectionType. You cannot call them unless your proxy is declared in a var, even though (I hope) none of these methods actually try to modify self.