Objective-C code:

@implementation MyGreatClass {
    BOOL safeBool;        // is allocated on the heap as part of each MyGreatClass instance, and gets set with an initial value of NO
}

- (void)myGreatMethod
{
    BOOL wildBool;        // is allocated on the stack, and the memory for it is not cleared first so the initial value will be whatever previously occupied that same memory location.
    if(wildBool) {
        // wildBool could be anything
    }
}