I quite believe it! Some years ago my boyfriend was given a 6ft tank with a few juvenile tropical fish in it but it looked bare so he he went to an aquatic store and bought a small school (about 6) of fish that he liked the look of. No advice from anyone and he didn't even know what the fish were that he'd bought as they weren't labelled. They were very small at the time he added them to his tank. Anyway, as time went on, those tiny little fish grew much bigger and he was curious what they were so he took a photo and went to his LFS.
The chap there told him the fish were Piranhas! My boyfriend didn't believe it. He told the chap that they couldn't be piranhas because there were a few other tropical fish in the tank and they'd have been killed by now. The LFS chap said it was impossible...no way could there be other fish in with them as they were definitely piranhas. So my boyfriend went back the following week with a photo of all the fish in the tank together. The LFS chap was speechless and didn't know how that could be.
Anyway, whilst there my b/f decided to purchase another couple of small fish, thinking smugly that he had a special type of peaceful piranhas lol. He took them home, put them in but was shocked to see within seconds of releasing them, one of the new fish floating in half. Then he saw one of his piranhas swim past the other new fish and that, too, was killed by the piranha.
He was baffled. Why did they kill the new fish but not the others? He went to another LFS and told them the whole story. They said it was because the piranhas were babies when they were put with the first lot of fish and grew up with them, accepting them as part of their group. Whereas adding any new fish would be seen as food...
Very strange but interesting, I thought.
Athena