What's the biggest financial mistake you see people make all the time?

Buying something that they can’t afford. Too many times I’ve seen people handing over shiny credit cards for their purchases without doing the maths to see if they can pay it all back. You’ll see fancy hatchbacks outside flats, bent and scratched by in others in jealously because of the carelessness of the neighbourhood. People just want to impress others by having the latest kit. They’ll live in squalor but they just want to show off. After all, it’s a material world. But before they know it, they can’t maintain it. The bill comes and they have to sell it on to pay their regular bills. The necessities like food and rent. So then it’s onto the pawn shop to make that quick buck to keep them going and absorb and delay the cost. In the meantime, they’ve still got to pay for it with interest; which someone else is making a living off of. So it always pays to know what the fee is going to be; and to disappoint them by paying it off before the due date.