in tables.c you'll find this function. notice the sn <= num_skills in the for loop everywhere else it is just < num_skills.

void remap_slot_numbers(  )
{
   SKILLTYPE *skill;
   SMAUG_AFF *aff;
   char tmp[32];
   int sn;

   log_string( "Remapping slots to sns" );

   for( sn = 0; sn <= num_skills; ++sn )
   {
      if( ( skill = skill_table[sn] ) != NULL )
      {
         for( aff = skill->first_affect; aff; aff = aff->next )
            if( aff->location == APPLY_WEAPONSPELL
                || aff->location == APPLY_WEARSPELL
                || aff->location == APPLY_REMOVESPELL
                || aff->location == APPLY_STRIPSN || aff->location == APPLY_RECURRINGSPELL )
            {
               snprintf( tmp, 32, "%d", slot_lookup( atoi( aff->modifier ) ) );
               DISPOSE( aff->modifier );
               aff->modifier = str_dup( tmp );
            }
      }
   }
}

I wonder what the point of that function even is, but good catch. Once someone managed to hit 500 skills on their MUD it would have gone beyond the end of the array.

FWIW, this was added in Smaug 1.4.

Lol I went and looked at LOP and it had that same issue lol.