This happened recently.
My mother and I were travelling to Bangalore from Calicut. She had a confirmed berth - but it was in the middle. At the time of reservation, I had no choice; there were no lower berths available. So out of the urgency of that travel I had to accept what the system auto-allocated. I did what any other Indian would do...exchange with other passengers for a lower berth so that she could sleep comfortably.
Fortunately, there was a group of women travelling along with their kids. They had their seats reserved in the same coach, but different, but adjacent compartments. When I requested they casually obliged. And the exchange happened peacefully. I made sure I exchanged with a party that was going to drop off in Bangalore. And they had confirmed tickets. I never bothered to check their tickets however.
But later, after probably Shornur station, the berths we (my mother and I) were occupying got allocated to another party (who were in RAC status). When the ticket-checker came and allocated, he did ask about the seats and my ticket. When he understood that I had a confirmed ticket, he asserted in the most callous way: I have allocated the berths...all of you please co-operate among yourselves and see that you are berthed. He never made an attempt to find out if the berths were claimed. According to him, he has diligently marked those berths as 'not allotted'. It was nearing Palakkad. The station where the ticket-checkers usually interchange as their shift comes to an end.
The people who were alloted the berths were students. I explained them the issue. I was sure that the checker made some mistake. Even they were sort of convinced. But we couldn't find the checker anywhere. He disappeared. Even those three didn't make an effort to find the ticket-checker. Finaly we agreed we'll clear it with the next checker.
And the time came, with a little trouble waking everyone up, we presented the tickets. Thats when I understood the problem. The lady who I had exchanged with presented her tickets. They were two tickets. But they were stapled together. So when she presented it, it looked as if she had presented only one ticket.
That diligent (b*****d) of a ticket-checker (who was in the previous shift) only marked the berths in the first ticket. He marked the ones in the other compartment (the one which I occupied, and which were in the second ticket) as not allotted. And he, obviously not guilty of the previous crime, further went and allotted them to three RAC passengers.
Well, the now on-duty checker thankfully realized this problem. And he gave consideration to the family and told the three students to kindly adjust. Luckily those three, only asked for one seat so they could adjust and travel. He also gave the story of how someone from S8 coach, without getting his ticket checked, went to join his family in a different coach, and finally got his berth marked as not-allotted. Thus problems ensued. I don't know how they resolved it.
Such screw ups happen inside of trains. I only pray the ticket checkers are more considerate...Hope this story has given you an idea of things to put in your checklist before exchanging berths.
Now what if you don't have a physical copy of a ticket? What if the ticket was just an sms? How will you ensure that the ticket was checked by the checker?