Nsgenericexception Reason Collection <Nsconcretemaptable: Xxx>

Collection was mutated while being enumerated error in objective C

Because your logic is flawed. It is not permited to mutate a collection during enumeration. And in the latter case, NSMutableArray doesn't know you're trying to emumerate it, only in the first case. And then it complains, since this is a semantic error. You should generally solve these kinds of problems by mutable copying the array and mutating the copy, then replacing the original one by the updated copy.

Error when attempting to remove node from parent using fast enumeration

Behavior may change between iOS versions. It may have actually crashed at some point or very rarely even in Xcode 5, you just didn't get to see it.

The problem is easily circumvented by delaying the execution of the removeFromParent method. This should do the trick because actions are evaluated at a specific point in the game loop rather than instantaneously:

[self enumerateChildNodesWithName:@"name" usingBlock:^(SKNode *node, BOOL *stop) {
    if (node.position.y < 0 || node.position.x>320 || node.position.x<0) {
        [node runAction:[SKAction removeFromParent]];
    }
}];

If this won't work use the "old trick": filling an NSMutableArray with to-be-deleted items and removing the nodes in that array after enumeration:

NSMutableArray* toBeDeleted = [NSMutableArray array];

[self enumerateChildNodesWithName:@"name" usingBlock:^(SKNode *node, BOOL *stop) {
    if (node.position.y < 0 || node.position.x>320 || node.position.x<0) {
        [toBeDeleted addObject:node];
    }
}];

for (CCNode* node in toBeDeleted)
{
    [node removeFromParent];
}

CoreData change relationships in a loop

The problem is that you can't change (mutate) a collection like your greenCategory.items set while you're looping over it, which is what you're doing when you change the category of the items in that set. What you have to do is create a separate collection and loop over it instead.

NSArray *greenCategoryItemsArray = [greenCategory.items allObjects];

for (Item *aItem in greenCategoryItemsArray) {
    [aItem setCategory:blueCategory];
}

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