Now my Xcode version is 6.1. But I got this warning too. it annoys me a lot . after search again and again.I found the solution.
Reason:You must have set your UILabel Lines > 1 in your Storyboard.
Solution: set your UILabel Lines attribute to 1 in Storyboard. restart your Xcode. It works for me, hope it can help more people.
If you really need to show your words more than 1 line. you should do it in the code.
//the words will show in UILabel
NSString *testString = @"Today I wanna set the line to multiple lines. bla bla ...... Today I wanna set the line to multiple lines. bla bla ......"
[self.UserNameLabel setNumberOfLines:0];
self.UserNameLabel.lineBreakMode = NSLineBreakByWordWrapping;
UIFont *font = [UIFont systemFontOfSize:12];
//Here I set the Label max width to 200, height to 60
CGSize size = CGSizeMake(200, 60);
CGRect labelRect = [testString boundingRectWithSize:size options:NSStringDrawingUsesLineFragmentOrigin attributes:[NSDictionary dictionaryWithObject:font forKey:NSFontAttributeName] context:nil];
self.UserNameLabel.frame = CGRectMake(self.UserNameLabel.frame.origin.x, self.UserNameLabel.frame.origin.y, labelRect.size.width, labelRect.size.height);
self.UserNameLabel.text = testString;