I get "found money" - money outside of our normal income - in a couple different ways. Birthday gifts and blog income are probably the most common ways that we get money in that's not normally expected.
For years, I have noticed that I have a terrible tendency to double dip my found money. For example, I might get $100 for my birthday, and I will buy a Groupon for $60 for a couple massages. Then I will splurge on an extra round of swimming lessons for Baby M for $50. Then I will treat myself to a couple of lunches out, "with my birthday money", for maybe $30. Suddenly, I've spent $140 of that $100! I forget each unusual purchase almost as soon as I've made it, so in my mind, the entire $100 remains unspent.
I used to have this problem with mystery shopping money, which was a big problem because I was actually doing that to help ends meet, so I couldn't afford to splurge that income at all. I finally ended up creating special budget categories that were ONLY filled with that money - so if I didn't put the mystery shopping income there, there was nothing to be spent on clothes for that month, or whatever it was.
Now I don't keep a strict written budget so much anymore, much less an envelope system, so that is a little harder to come by. I'm trying to pay closer attention to my habits, but I think next time I get "free money" I will take it out in cash and only spend it in cash - when it's gone, it's gone, and that will be the end of it.
Do you find that you tend to do some double accounting when it comes to found money?