Quick test to find out what happens if an NSMutableArray is changed significantly within a for…in loop. (Modified from the ViewController.m from Xcode's iOS Single View Application project template.)

ViewController.m

//
//  ViewController.m
//  LoopErrorTest
//
//  Created by Slipp D. on 5/13/13.
//  Copyright (c) 2013 me!. All rights reserved.
//

#import "ViewController.h"

@interface ViewController ()

@end

@implementation ViewController

- (void)viewDidLoad
{
    [super viewDidLoad];
	// Do any additional setup after loading the view, typically from a nib.
	
	NSMutableArray* array = [NSMutableArray new];
	[array addObject:@"1"];
	[array addObject:@"2"];
	[array addObject:@"3"];
	[array addObject:@"4"];
	
	int loopCount = 0;
	for (NSString *element in array) {
		NSLog(@"element %d: %@", loopCount, element);
		
		if (loopCount == 2) {
			[array removeLastObject];
			[array removeLastObject];
			[array removeLastObject];
		}
		
		++loopCount;
	}
	
	[array release];
}

- (void)didReceiveMemoryWarning
{
    [super didReceiveMemoryWarning];
    // Dispose of any resources that can be recreated.
}

@end