Some friends are planning a wedding, and they've set themselves a very strict budget. I'm not sure of the amount, but it turns out that everything they would like is just a little too expensive for what they have to spend.
I think there could be two things going on. One is that it's so easy to do little upgrades. If you go to Starbucks and ask for a small coffee, they offer a medium for only $0.26 more. You would be surprised how many people say yes to this. Even if all they wanted was a small coffee in the first place (which I assume is true, since that's what they ordered!) they will jump at the chance to "upgrade" - even though it costs them more. (Fun thing to do on a Saturday afternoon: sit near the register at a Starbucks and count the number of people who accept or decline upsizing. Wait, did I hear you say that I have no life?) I don't often fall for upsizing. I worked in the food industry for too long, I mystery shopped for two long, and I am generally too cheap. And it is pretty easy to avoid - simply say no to upsizing, and stick with what you originally wanted - whether that's a small coffee, a two-entree buffet instead of a four-entree buffet, or a used car. (This is why it's really, really, really important to only look at houses within your pre-arranged budget. Seriously.)
The second thing that might be going on with my friends is something that I have personally experienced a number of times. That's when you set a budget that sounds realistic but actually isn't. Either you didn't do the research ahead of time, or there was no way to predict how much it might cost for your situation, or something else has intervened to mean that the reality is that what you have to spend simply isn't enough.
This happened with our honeymoon. We weren't going to go on one at all, and then through some hardcore savings we found ourselves with an extra $1,000 or so - and I thought, Oh, we can do a honeymoon on that, no problem! I did a little research that seemed to indicate that was true, and off we went.
Well, it turns out, it cost us a bit more than $1,000 to go to Niagara Falls for our honeymoon. Not a ton more, and we were able to pay cash for the difference, but if this type of thing happens on a regular basis, you can see how you're in for a world of hurt. In my case, I had done some research but I had not paid close enough attention to the exchange rate between the US dollar and the Canadian dollar. I also didn't look into how much food cost in such a touristy area - the price of transportation and attractions was easy to figure, but the meals are what did the budget in.
The best way I know to avoid this second form of budget creep is two-fold. For one time or unusual purchases, like a wedding, do a lot of really in-depth research. But if everything seems to be costing just a little bit more than you expected, try tracking your spending. Once you see how much you already are spending on food, entertainment, gas, or clothing, you'll have a better idea of where you can cut back.