Due to and owing to

Mr Sleigh was the senior English master at my school.  A tall, bald, imposing figure, he was a strict disciplinarian and was universally known as "Killer," in reference to the homophone 'slay.'  

"The difference between 'due to' and 'owing to,'" he told us, "is that 'due to' always follows the verb to be."  Thus "my train was late owing to a signal failure, but the lateness of my train was due to a signal failure." 

At this point my friend David Middleton raised his hand, with the pocket Oxford English dictionary that we all kept for English lessons open on his desk.

"Please sir, in the Oxford English Dictionary when you look up 'due to' it says 'owing to,' and when you look up 'owing to' it says 'due to,'" he informed the teacher.

Mr Sleigh stared impassively.

"There is a very simple explanation for that, Middleton," he replied.  "It is because the Oxford English Dictionary is WRONG."  The last word was delivered with such great force that the whole class flinched.

I suppose that was one of my early lessons in learning never to accept authority unquestioningly.