I took my iPhone in since it suddenly stopped working properly. It suddenly started acting like the headphones were in all the time. I bought it 9 months ago, so still under warranty.
I took it into my local telco who tell me to wait while they check it out. After 20 minutes they come out with the phone and tell me, oh no it's not going to be covered since it's water damage (I legitimately hadn't damaged it with water) and showed me the marker at the bottom of the port. I asked them for the phone back then, fuming that I now had a super expensive iPod touch and would now need to buy a new phone.
They handed it over to me and I checked it over. For no particular reason I decided to look at the back where I have a fairly distinctive mark that comes from my case. The mark was gone.
I unlocked the phone and saw it was a completely different one, none of my apps or contacts were there. I asked them about this and they said (actual quote), "Yeah we accidentally wiped the data when we were investigating your issue."
I asked them what they did exactly and they claimed to have pressed a reset button in the headphone jack. At this point I demanded to see the manager and told them what had happened. He didn't believe me until I told him about the "reset" button that didn't exist.
Turned out the guy who was dealing with the problem was finishing that day and was trying to get as many phones as possible to try and sell them later. I have no idea how he planned on getting away with it.