I've found a behavior discrepancy between simulator and device (iPad) when I use some graphic functionality and Grand Central Dispatch.
dispatch_apply(count, queue, ^(size_t i) {
dot = [dots objectAtIndex:i];
CGRect rectangle = CGRectMake(dot.x,dot.y,15.0f,15.0f);
CGContextSetFillColor(currentContext, CGColorGetComponents(dot.color));
CGContextAddEllipseInRect(currentContext, rectangle);
CGContextFillEllipseInRect(currentContext, rectangle);
CGContextStrokePath(currentContext);
});