rats can be sexed from they are a day old. With 14 week old rats there's no way they should be making a mistake.
I got my first rats from Pets at Home 7 years ago, so if they have been selling rats for so long they should know how to sex them by now. It didn't matter what sex the rat was, I wanted it either way, I just didn't want a female that had been living with males because they thought she was a male for some reason. If the mistake hadn't been made and someone had said oh that's a female then they would have just said yes and we would have been none the wiser until babies had appeared, that would have been quite a shock! Pets at Home should at least check their animals when they come in so that they can be put in single sex groups, they should also be checking their health, not just their sex, if anyone had been checking her on a daily basis they would have realised she was a female and was in with males, they mustn't bother with their animals at all.
At least it wasn't some kid who bought the rat only to end up with babies, they would have had no choice but to give it back to the petshop. But the little rascal will stay with me, she's already escaped from her cage twice