One of the rules in Swords & Wizardry Complete is that wizards must have a certain Intelligence score to cast spells of certain levels, as per the Intelligence table in the rulebook. A house rule has been added to help alleviate this problem - wizards can cast higher level spells than their Int score allows, at a risk. When casting a spell of a higher level than his Int score would normally allow (provided he has been able to learn it successfully - see below), the wizard must make a saving throw with a penalty equal to the difference in level of the spell being cast and his normal maximum spell level. For instance, a wizard with a maximum spell level of 6 (Int 12) casting a 9th level spell would make his saving throw at -3. If he fails this saving throw, he suffers 1d6 points of damage per level of the spell as the magical energy backlashes due to an incomplete understanding of how to harness it.
Learning Spells of Higher Level Than Intelligence Allows: Wizards may learn spells that exceed the limit of their Int score, but it is difficult. A -5% penalty per level higher than the normal maximum spell level limit is applied to the wizard's chance to understand a new spell. Thus, a wizard whose maximum spell level limit is 6 (Int 12) trying to learn a 9th level spell would have a 40% chance to learn it, instead of the normal 55%.