When I first started working in the radio business, a disk jockey actually played records. Before your shift began you would go to the music library and pull the singles and albums you were going to play. At some of the more heavily formatted stations you had to make sure a certain percentage of the songs you played were from the program director's playlist. As time went on, playlists became more enforced, until finally you had a list of songs that you HAD to play, though the disk jockey could still use his or her discretion as to what songs were played in what order.
As I was graduating from college, the radio station where I worked had started automating the song selection process. Our radio station had 3 large reel to reel tape decks. Every week we were sent a box of reel to reel tapes. The disk jockey would mount a tape on each tape deck. There was a sheet of paper for each tape that listed the songs. You would tape the 3 sheets of paper to the control board, so you could see which songs were playing next. You followed a format that specified which tape deck the songs would be played from (e.g. 1-2-1-3-1-2-1-3-2-1-2-1-3 repeat). You were not allowed to skip a song or play the songs out of order. We all skipped songs and played the songs out of order. Sometimes a disk jockey would step out for a break (smoke, bathroom, munchies, groupie) and just leave a tape playing through 2, 3, 4 songs.
I haven't been inside of a radio station's control room for several years. From what I understand, everything is completely automated now. The disk jockey has virtually no control at all over what goes over the air. That's how incidents like the one Tuesday occur. A radio station in Charlotte, N.C. found itself playing "Another One Bites The Dust" right after reporting on the Washington area sniper. The producer of the radio broadcast claims "the song was inserted weeks ago into the show's computerized log for play at 6:40 a.m. Tuesday, the first music track coming out of the newscast." Talk about bad timing.