Not building a financial cushion
Money doesn't buy happiness but it does give you options and breathing space.
If you don't have a little money put by, you may not be able to move house, travel further to work, buy a more reliable vehicle, do that university course or tide your partner over until they can find a new job.
Any of these and more may stand in the way of your next crucial career move. Then, looking further ahead, you may find yourself unable to retire early or at least take less responsibility in your role.
What is the right thing to do? Yes, it's difficult. We have all been there, with babies and a mortgage and the future another country. But hand on heart, with hindsight, we could all probably have saved a little better, earlier and more regularly. Compound interest isn't just a maths problem.