Background queues sometimes execute their blocks on the main thread.

DispatchQueueThreads.m

#import <Foundation/Foundation.h>

int main(int argc, const char * argv[])
{

    @autoreleasepool {
        
        dispatch_queue_t q = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
        dispatch_sync(q, ^{
            NSLog(@"Sync on main: %@", [NSThread isMainThread] ? @"YES" : @"NO");
        });
        
        dispatch_queue_t q2 = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
        dispatch_async(q, ^{
            NSThread * origThread = [NSThread currentThread];
            NSLog(@"Async on main: %@", [NSThread isMainThread] ? @"YES" : @"NO");
            dispatch_sync(q2, ^{
                NSLog(@"Inner sync on main: %@", [NSThread isMainThread] ? @"YES" : @"NO");
                NSLog(@"Inner sync on same as async: %@", [[NSThread currentThread] isEqual:origThread] ? @"YES" : @"NO");
            });
        });
        
        dispatch_main();
        
    }
    return 0;
}

This is very interesting - I get:

Sync on main: YES
Async on main: NO
Inner sync on main: NO
Inner sync on same as async: YES

Yes indeedy; those are the expected values, @rsaunders100. The queue avoids doing an expensive context switch in the "sync" case, because the originating thread has to wait anyways.