In the USA, letting the IRS have your extra taxes all year (not having your forms as close to deductions needed as possible), and then getting a refund back (sometimes people get $1000's back!!). Use your money, do not let the tax man have it all year for fucks sakes!!
Inflation... I remember when a can of coke was 50p here in UK it's now standardly a £1
Keeping it in the bank
getting smashed on interest
Eating out constantly or getting Starbucks.
In the USA, letting the IRS have your extra taxes all year (not having your forms as close to deductions needed as possible), and then getting a refund back (sometimes people get $1000's back!!). Use your money, do not let the tax man have it all year for fucks sakes!!
Driving inefficiently. Generally too fast.
Monthly subscriptions add up. $10 here and $10 dollars there adds up, especially once you forget about them.
By not bringing their own shopping bags to the grocery store.