I would do a factory reset on the game, the set up freeplay and balls per game etc and see if it happens again/anymore. Also, make sure the game has fresh batteries in the battery holder.
If it does happen again, I would get a new set of roms burnt/supplied and make sure the roms have the latest version that is available (check IPDB).
I cant see why this wouldnt fix the issue.  One of the other AP guys will add something If I have missed anything. Hope it helps.  

* VIDEO ADDED 4 YOU *
I have added that excellent factory tour VIDEO of Gottlieb below when the MYSTERY CASTLE machine was getting made by the hundreds.
I saw that another AP member posted it recently. If u havent seen it before, its a great video.