I hate to tell players "no" when they want to try something, and here's a common example. The heroes uncover a treasure trove and inside is a book of magic spells. Upon studying it, the wizard discovers it contains a few spells he already knows, a few he can understand and learn, and a few that are technically a higher level than he can cast.
Can't he try to learn them anyway, and have a chance at figuring them out? Why, sure he can! Here's a simple system to let your Microlite74 magic-user learn and cast spells of a higher level.
Learning a higher level spell: DC=10+level of spell times 2
Magic user rolls d20+level+Mind Bonus
Results greater than or equal to the DC means he has learned the spell and can now cast it. However, until he gains a high enough level to normally be able to cast the spell, it costs twice as many hit points to cast.
Results less than the DC means he cannot learn this spell until he has gained the proper level.
Results of five less than the DC means the spell backfires while he is attempting to learn it (use your favorite backfire/misfire table).
Results of ten less than the DC means the spell backfires catastrophically while he is attempting to learn it (use your favorite backfire/misfire table and the magic-user is unable to cast any spells for a number of days equal to the DC).
Rolling a 1 is an automatic catastrophic failure.