The case
Two magazine sweepstakes ran dueling commercials through the '80s and '90s. Ed McMahon, Johnny Carson's sidekick, fronted American Family Publishers with the line about "you may already be a winner." Publishers Clearing House had the balloons, the van and the oversized check.
Because both pitched the same dream in the same commercial breaks, the most famous face got attached to the most famous check. McMahon himself spent years correcting it.
Why our brains do this
Two similar campaigns share one memory slot, and the most recognizable element of each gets combined.
Sources: Wikipedia — Ed McMahon · Wikipedia — Publishers Clearing House · Wikipedia — False memory: the Mandela effect