We spent $34,500 in 2014 and budgeted $32k. That $34.5k included an $8,700 major renovation that won't happen again for 20+ years (new siding and windows plus major roof repair). Otherwise we would have been well under budget.
I intentionally put some fluff in the budget to cover unexpected expenses and increases over time as the kids get older. And I didn't properly account for getting more efficient spending in ER (groceries, for example).
We'll probably underspend in 2015 since we won't take any big trips other than maybe a cheapo road trip and maybe a cruise to the Caribbean ($3000-3500 total). With $5300 budgeted for travel, we probably won't use it all up. There's a 2 year old involved, so traveling isn't as much fun as it will be when he's older.
Long term, I'm expecting a 90% chance of having more money than we know what to do with and a 5% chance of feeling a real budget crunch from poor portfolio performance. Such is the way of planning for the worst knowing it's not likely to come true (4% rule adjusted to 3% for age 30-something retirees).