Most of us are guilty of justifying a purchase with, "I deserve it." Unless you budgeted it for it at the beginning of the month, you don't.
Even though you may have had a tough week or just accomplished a large project, you owe it to yourself to delay gratification. The momentary satisfaction of buying something isn't worth going outside of your budget and the havoc it can create.
Dave Ramsey had a story about a single mother giving in at the end of a tough day and buying McDonald's for her kids even though it wasn't in her budget. Those happy meals ended up costing her over a hundred dollars due to the NSF fees from both her bank and the check payees. In the future I'll try to remember this story when I try to justify a purchase by telling myself that I deserve it.