Buying clothes without trying them on, continuing to drive and pass by exits when the gas light is on and deciding whether to eat something or not based on smell.
These are three things you've probably guessed at some point. How'd that go for ya? Yeah, which is why you shouldn't guess on your budget either.

Lifehacker says the “adulating” budget doesn't include copying and pasting the same amounts from month to month.

Instead, really think about what is happening that month -- are you taking trips? Having people over for dinner? Do you need to buy clothes for a changing season?
So what if something changes? And it will. Build in a buffer, even it's just a small one.

And make adjustments mid-month. If you're overspending in one category, you have to cut back in another. The key is if you say yes to one thing what do you need to say no to. You choose!