you probably didn't include clones.
pacman is a clone. puckman is the parent.
if you're using merged sets, you'll be required to have the parent.
What he said.
A word of advice to those that are tyring to save space by removing clone roms...... don't do it.
Many of your favorite games are actually clones of a japanese parent, especially if you want the US versions. If you are running a split set (most common) then the extra space the clones take up is negligible. Remember only the different roms are in the clones, which usually equates to a puny language file and perhaps different graphics for the title screen.
Just some examples of where you want to use the clones:
Pacman
Ultimate Mortal Kombat 3 (clone of regular mk3)
all the konami beat-em-ups.... the 2 player versions of the games (which you'll most likely want to play) will be clones of their 4 player parents.