bad brake install

Hello,

I recently changed the front brake pads in a family members 2011 Nissan Sentra base model. I did this because I was hearing a squeeling noise coming from the front drivers side wheel. I thought it might be a brake pad wear indicator because the noise seemed to stop when I applied the brakes. I haven't changed the pads in a car in quite some time, but I thought I would be able to manage it. I thought I did a decent enough job and when I took the car out to test the brakes, they worked great. However, after a bit of driving the noise came back. Its at this point I decided a mechanic should take a look. Another family member took the car to the shop and when they got back I was a little surprised. The shop had said that the brake pads where installed improperly and that they were no longer useable. They ended up replacing the brand new pads with different, more expensive ones as well as changing the rotor and they send the pads I had put in back with my family member and told them to hold on to them for the future (which I don't understand why, if they are no longer useable). They also did some other things which I'm not sure of, but the noise is no longer there.

My question is, how can you screw up a brake pad replacement so that the pads are unusable? I would basically like to know if we just got ripped off at the shop. Obviously it is hard to know exactly what was done, but I'm just wondering, in general, what are the things that can be done during a pad change that could cause them to be installed incorrectly.