We have some exciting news to share! The Motley Fool UK has now become The Twelfth Magpie -- an independent, UK-owned company, led by our long-serving UK management team — Mark Rogers, Chris Nials and Heather Adlington. In practical terms, it’s the same team you know, now fully focused on serving our UK readers and members.

Just as importantly, our approach remains unchanged: long-term, jargon-free, and on your side. This site is our new home, and there will be extra tweaks made across the coming few days as we settle in. So if anything looks a little off, please bear with us!

The content of this article was relevant at the time of publishing. Circumstances change continuously and caution should therefore be exercised when relying upon any content contained within this article.

5 investing mistakes to avoid

A long-term investor reviews five potentially costly investing mistakes that could hurt his portfolio. Learn what they are and why they can matter.

A young woman sitting on a couch looking at a book in a quiet library space.

Image source: Getty Images

You’re reading a free article with opinions that may differ from The Motley Fool’s Premium Investing Services. Become a Motley Fool member today to get instant access to our top analyst recommendations, in-depth research, investing resources, and more. Learn More.

As an investor I have made mistakes. Some have been costly – but also valuable. Investing mistakes have helped me learn how to act more carefully in future.

Here are five potentially costly investing mistakes I now focus on avoiding.

XXX

Investing mistake 1: too much focus on one share or sector

Imagine if I had bought Apple, Amazon or Greggs when they were relatively cheap and ridden them a long way up. I would have been pleased with my return. But I also might have stopped looking elsewhere.

Diversification is a crucial risk-management tool in investing. I know that. But if I think a stock has strong prospects, it can be tough to remember why diversification is so important.

That investing mistake can be costly. No matter how attractive a share or sector seems to be, no one can guarantee future performance. What if I’d bought shares in just a few ultimately unsuccessful companies? I’d be a very unhappy investor.

Investing mistake 2: not doing my homework

Buying shares can be like purchasing a car or house. Whatever the external appearance, it pays to look closely at the details.

A good example of that in recent years was roadside assistance group AA. At first glance, this would have struck me as attractive. It had a respected brand name, a strong business model and significant cash flows.

But looking at the annual accounts would have revealed the company’s huge debt pile. The company was taken private again this year at just 35p a share – leaving some shareholders 90% down from their 2015 purchase price.

Fortunately I did not buy AA. A quick look at its balance sheet online was enough to put me off.

Investing mistake 3: leaving my circle of competence

As an investor I do not worry about only understanding a few industries. But I do worry about stepping outside my circle of knowledge.

It is a common investing mistake to chase higher returns by investing in companies or industries one does not remotely understand. But legendary investor Warren Buffett fastidiously sticks to what he knows. I’m following his example.

Investing mistake 4: ignoring my investment strategy

I find it helpful to have an investment strategy. This can start very simply and evolve over time. But it helps me remain clear about what I want to achieve through investing. It also focuses me on applying consistent principles as to how I will try to achieve my goals.

I admit it can be tempting to ignore it. In the past few days, for example, I weighed opening a position in Bacanora Lithium. But I followed my investment strategy and decided against that move.

My strategy is not fixed forever. I set it and can change it. But defining my strategy and sticking to it helps me discipline my investment choices.

Investing mistake 5: encouraging confirmation biases

There are risks in tobacco shares, not least the decline of smoking in key markets. As a holder of British American Tobacco and Imperial Brands, it could be easy for me to downplay that. It’s always comfortable to confirm what one thinks, for example by cherry-picking information sources.

To help avoid that investing mistake, I always try to see the other side of the investment case on any shares I consider. That can be painful – but hugely helpful.

John Mackey, CEO of Whole Foods Market, an Amazon subsidiary, is a member of The Motley Fool’s board of directors. Suzanne Frey, an executive at Alphabet, is a member of The Motley Fool’s board of directors. christopherruane owns shares of British American Tobacco and Imperial Brands. The Motley Fool UK owns shares of and has recommended Alphabet (A shares), Alphabet (C shares), and Amazon. The Motley Fool UK has recommended Imperial Brands and recommends the following options: long January 2022 $1920 calls on Amazon and short January 2022 $1940 calls on Amazon. Views expressed on the companies mentioned in this article are those of the writer and therefore may differ from the official recommendations we make in our subscription services such as Share Advisor, Hidden Winners and Pro. Here at The Motley Fool we believe that considering a diverse range of insights makes us better investors.

More on Investing Articles

Friends and sisters exploring the outdoors together in Cornwall. They are standing with their arms around each other at the coast.
Investing Articles

£503 buys 14 shares in this FTSE 250 stock that returned 23.9% annually for the last 15 years

This FTSE 250 stock has averaged a huge return for 15 years. At today's price, £503 buys 14 shares. But…

Read more »

Black woman using loudspeaker to be heard
Investing Articles

£1,000 buys 25 shares in this FTSE 100 stock that’s returned 29.2% annually for the last 10 years

This FTSE 100 mining stock has returned close to 30% a year for a decade. At 3,995p, £1,000 buys 25…

Read more »

Female student sitting at the steps and using laptop
Investing Articles

Down 47%, is this growth stock finally worth buying in May?

With a £288m order book and a hidden pipeline of defence and nuclear contracts, is this growth stock now too…

Read more »

House models and one with REIT - standing for real estate investment trust - written on it.
Investing Articles

2 REITs yielding 7%+ to consider for passive income in 2026

A REIT backed by the NHS and another backed by Tesco and Sainsbury's with both yielding 7%+. Here's why I'm…

Read more »

Woman riding her old fashioned bicycle along the Beach Esplanade at Aberdeen, Scotland.
Investing Articles

Just 97 shares of this UK dividend stock generate £238 in passive income

A 5.7% yield, £238 in passive income from just 97 shares, and one of the most divisive dividend stocks on…

Read more »

ISA coins
Investing Articles

£10,000 in an ISA generates a second income of…

The London Stock Exchange is home to some of the world's most generous dividends. But how big a second income…

Read more »

Shot of a senior man drinking coffee and looking thoughtfully out of a window
Investing Articles

Expert recommendations: 2 top income stocks yielding 7%+!

With yields of 7.2% and 7.8% respectively, these two income stocks are catching the eyes of institutional analysts. Should investors…

Read more »

Illustration of flames over a black background
Investing Articles

3 top income-focused stocks to buy in May 2026, according to experts

Looking for a stock to buy for income in May 2026? Experts have flagged these three UK dividend shares as…

Read more »