I've had two problems with two sets that I have bought in the past. The first is the Clone Turbo Tank, in which instead of using a 2x2 upward slope two 1x2 upward slopes were included, so I asked them to fix this problem. There was an extra little sheet of paper inside the box that told me to substitute the 2x2 with the 1x2's but I felt that I wanted the set like it was really supposed to be. The other problem with this set was that instead of getting 1x2 trans-light blue grilles, I got just normal light blue grilles. I asked them probably 5 times for trans-light blue, but i kept getting normal light blue, but finally they sent me the correct ones. Now this is definitely GREAT effort by TLC to satisfy customers. I really thought they would just think I was trying to steal pieces, but it turns out that they kept sending the pieces to me until I got the trans pieces. Kudos to Lego.
I think the reason for this was because they made a mistake in parts production or just ran out of trans-light blue grilles.
The other problem was with my V-19, in which I never got a sticker sheet. I asked customer service for a sticker sheet and there it was in my mail, only about 10 days after I requested it.
As far as mangled pieces go, I haven't experienced that. The only thing that occasionally happens is when the minifig printing is pretty off, like in my old ep2 clone from the 7163 RGS. The helmet is printed from two sides and the black printing from the left and right side of the helmet did not connect together in the middle. But overall it looks fine and there's no reason to have to ask for another clone.
But anyways, I really appreciate TLC's efforts to be as perfect as possible as they have already done such a good job.
I wonder if I could get replacement stickers for my 7163 RGS that peeled off like 3-4 years ago...?
By the way, the stickers from newer sets (starting from my 4504 MF) really do seem higher quality as I have not experienced any problems with them