Only once - and it was a major problem! About 5 years ago I was doing a 7 day rover with some friends, and part of the trip was a sleeper journey back from Penzance to London. Now, it came to light during the holiday that I had left all the sleeper reservation coupons, vouchers, tickets etc etc etc at home. After a few phone calls we were assured it would all be ok, all we had to was present ourselves at Penzance, give our reference code ( which I still had) and they would cross reference with the list they carry of who is booked and hey presto.
Very unfortunately there had been a ferocious thunder storm which had resulted in a power cut in Penzance. Whilst the signals etc were working - the station was in darkness, and none of the computers were working. Result - no list.
So we had to sit on the cushions until Plymouth when the stewards decided they would probably have enough bunks to let us have one, when we finally got to sort ourselves out.
Now that is a personal tale - but at work I hear the same tale over and over again every single day "I've lost my ticket but I've got the receipt". Some are genuine, some not, you can never tell - so they all get the same treatment. Buy a new ticket, but if you happen to find it let me know. They will always winge, but my stock answer is "Imagine today you did some shopping, then left one of the bags containing (insert suitable item) on the train on the way home and it was never found. Would you go back to the shop the next day with the receipt and just expect them to hand over another (insert item) to you free of charge? No. Well this is exactly the same - you loose something so you need to buy another one."