Spending a month's pay in 2 weeks and use credit cards to make up the shortfall. This creates a cycle of debt which is very hard to escape from.
Borrowing beyond your means, without having any emergency fund.
Not protecting against poor health or premature death.
Not maximising pension contributions, especially those paid by your employer.
Believing investing is "risky" and not being willing to do any research to learn about it.
Buying depreciating assets such as cars and electronics rather than appreciating assets such as equities and property.
Not accounting for your tax bill if you are self-employed.
Trying to keep up with the Jones.
Elevating the importance of material items over experiences.
No drive for self-improvement.
Blaming others for your investment and spending choices.
Not realising that the bulk of "rich" people are self-made.
Not understanding investment market volatility and doing the wrong thing when the markets drop.