Changing out receptacle that is controlled by a light switch
Because you didn't look at the old receptacle to see if any tabs were broken off. 

You should break off tabs in the new receptacle exactly as you find them in the old receptacle.  

Breaking off the tab changes the meaning of the two screws. Instead of "two screws that connect to the same thing and can be used for supplying power onward to other receptacles", it now means "each socket in this receptacle is being fed power separately by a different wire, and each socket gets only one screw".  This is used for splitting receptacles for lamp control, and a few other uses. 

I have no idea if the old receptacle broke tab off the hot side (brass), the neutral side (silver) or both.  You will need to trash-dive the old socket and have a look. Breaking the hot side is the most common.  But if neutral had been broken/split before, and you fail to split it now, it creates a "silent but deadly" failure mode that will be revealed only by a fire or GFCI trip.