There's a great (though vague) section in the DMG describing spell completion items. It describes what happens when a character who isn't meant to cast a spell through a scroll or similar means and doesn't have the training and experience to do it right. The result is Scroll Mishap--a table of fun and delightful disasters that could occur as a result. The examples are fun and entertaining as well as providing interesting setbacks.
I read this and wondered--what if this is how it was with all magic? What if the answer to "what happens when a fighter tries to cast magic" or "what happens if my wizard tries to prepare a spell higher than the levels he knows" wasn't just that he can't--it's impossible, so tough!
What if the answer was, well you can...but here's why nobody ever does. And that way if they still try and do it, you make them roll versus spell mishap to see if they cause something unexpected to happen. If they do, they find out why nobody ever does that.
I've always liked the idea too of using the Spell Failure percentage roll for trying to memorize extra spell "slots" more than you can per day. For each one extra you try to prep you take a Spell Failure hit of 10-Int. mod. per spell for every spell you cast of that level that day. You don't make the roll, the spell miscasts or maybe it mishaps. That's the fun unexpectedness of magic.
It's certainly a heckova' lot more fun then "you just can't!"
I like it and if I remember when I start a new campaign I might just include it as a house rule