If you go over your limit, Chase can cover the charge for you as a "courtesy" feature - for a fee. These fees can be $39, even if you only go over your credit limit by a few dollars.
To avoid these fees, you can request that they turn off the courtesy feature. Simply call Chase and ask for an over-the-limit block.
If you do so, you won't be able to fudge on your credit limit anymore - but you should really try not to do that anyway, right? Instead of covering the charge, your card will be denied. You'll have to find another way to pay or decide to do without.