It is one of the most common conceptions, that goldfish have one of the shortest memories of all animals, with the memory level supposed to be that of a second. This is apparently a misconception, with the scientific belief being that goldfish can remember things for 3 months. However, when you combine this with the strongly held belief by some burglars that they need to get rid of all witnesses, the consequences were tragic for the goldfish.
Some teenagers robbed a house, and being young and enthusiastic, and having probably seen too many episodes of CSI, they were worried about being identified and caught, and hence decided to eliminate any possible witnesses to the crime, which in this case, were only some poor goldfish in the tank (link to article):
An Illinois teenager is facing residential burglary and cruelty to animals raps after killing a trio of potential witnesses to an Arlington Heights burglary - three goldfish. On 24 January, the unnamed 16-year-old and two accomplices, aged 15 and 17, targeted an apartment in a block which had been evacuated following a fire on 19 January. Police say they "pried open a door and stole a video game system, CD player, 30 video games, 30 DVDs, a BB gun, jewelry and a safe".
When the residents returned, they discovered "hot sauce, mustard, ketchup and spices had been poured into their fish tank and three goldfish floating at the top", the local Daily Herald reports.