Phoenix Dracul noticed that if player1 is an apprentice to player2, and player1 somehow loses the master section in the pfile, when player2 goes to promote him it will crash the mud because player1 doesn't have a master at that point.
in fskills.c in fskill_promote find:
send_to_char("You can only promote your own student.\r\n",ch);
before that add:
if( !victim->force_master )
send_to_char("They don't have a master\n\r", ch );