The way in which we think about our budget is too overly ordinary. It's as if we think we get up and do the same day over and over again: the same meals, the same commute and most importantly, ZERO unexpected activity.
Unfortunately, this predictable financial agenda is just never the case. For starters, there are expenses that we know we need but don't incur on a very regular basis. These might include: air travel, apartment furnishing, new shoes, golf rounds, Greek life events or concert tickets. We tend to adopt a take-them-as-they-come mindset for these and assume that they'll somehow just be absorbed. In reality, they should be included in a rollover budget.
And then there are the truly unexpected expenses that our brain assumes we'll never incur but our bank account knows will happen again. These might include: parking tickets, a new backpack, gifts for friends, car maintenance or apartment fines. If you decide not to budget for these, you should account for them through a short-term savings account with a balance strong enough to buffer even the heaviest of expense shocks, such as a $1,500 dental bill.