Here’s how you can learn from my five biggest investment mistakes

While you’re investing for that wealthy retirement, you’re sure to make some mistakes. But you can minimise them by checking up on the mistakes of others.

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.

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.

Earlier this year I shared with you what I considered my biggest investing mistake, but it was as much a commentary on the real value of long-term investing as anything. Today I want to share some specific mistakes, ones which in hindsight make me look like a bit of an idiot — but if it helps you avoid the same traps, I can live with that.

I’ll get my most embarrassing one out of the way first…

Borrowed money

I have invested in shares while simultaneously being in unrelated debt. And there was even a little bit of credit card debt thrown in too. Yes, bad, isn’t it?

If you invest, say, £1,000 in shares, you’ll probably have to pay around £10 in fees, plus 0.5% stamp duty, and you could easily face a spread of 1% too. So you’d have to make a return of around 2.5% just to reach the point where you’d break even if you sold. 

But suppose you have debts costing you even a modest 5% per year in interest. You need a return from your shares of 7.5% in your first year to break even, then 5% per year just to stay afloat. With the FTSE 100 averaging something around 6% per year long-term, that’s cutting it fine — never mind in the past five years when it’s grown by just 18%.

And if you’re holding credit card debt too, which could be costing you up to 30% in interest, you have no chance.

Short-term cash

A few years ago I had a bit of money set to one side, which I intended to use for something in about six months time. So did I just leave it in a bank, earning practically no interest but at least safe for when I needed it? Nah.

Thinking that over the long term, even short-term ups and downs with short-term cash would even out and I’d probably come out ahead. But that doesn’t actually help if you mess it up once and lose some cash that you actually need.

So I went ahead and bought shares with it, but you know when that was? It was not long before the financial crisis started to unfold. And guess what I bought? Barclays. I was fortunate that I sold again before the worst depths, but I did lose about half my money — and I lost it stupidly, by ignoring a key investment rule.

Investing in shares is way better than cash, but not with short-term money you know you’re going to need soon.

Understanding the business

Many years ago, I followed a PEG-based approach to finding growth shares. That in itself isn’t so bad, but my big mistake was to allow one of my automated rules to override common sense. I used to screen for a relatively modest forecast P/E multiple (I can’t remember what figure I used now) and PEG ratios of 0.7 or lower based on two years of earnings growth forecasts — where the PEG relates the P/E to the expected growth rate. And then I’d pick the tastiest looking figures from what came through.

I bought shares in a company that ran a chain of nightclubs. It prompted a friend to say: “You know how to pick a share that’s already gone up, don’t you?

Yes, the price had soared over the previous two years. And yes, it subsequently collapsed again and I lost a chunk of money. What I’d neglected was the nature of the business. It was in a very competitive market and, more importantly, a market very much subject to fad and fashion — and the fashion moved on.

Trying to time it

In September 2015 and after watching the oil price slump, I concluded that it just couldn’t keep on falling forever. I was obviously right on that point, as were all the rest who understood that simple fact. But I made the mistake of thinking that I had some idea of when the bottom might be… and that we were at or near it.

So I bought some Premier Oil shares at 99p, and then watched the oil price crisis continue and saw my shares crash to 19p apiece… and then trading was suspended.

Since then there’s been a recovery to 130p and I’m actually now in profit, and looking at the share price today I could try to convince myself I made a good call. But I didn’t. That investment survived by pure luck and it had nothing to do with my skills at all. The fact was, Premier Oil faced a very real chance of going bust, and if the oil price had stayed down there for much longer I could easily have lost the lot.

A fortunate result, but a very silly move trying to time the market.

Diversify

I do believe that diversifying just for the sake of getting into a variety of sectors is a bad idea. That’s down to the fact that an awful lot of investors will often buy a stock that they wouldn’t buy if they weren’t trying to diversify. That is a mistake, and it is one that I’ve never made — you should never buy a share for any other purpose unless you would buy it on its own merits alone.

I’ve seen investors buying 15, 20 or more individual stocks, plus a few investing funds, purely for diversification — and a lot of professional advisors get rich by recommending just that. But isn’t that the perfect recipe for achieving… average? If you’re going to do that, I say just buy an index tracker and save all the time and fees.

But having said all that, I’ve rarely held more than four or five individual stocks at a time, and right now I own just four — and two of those are in the financial sector. While I don’t rate this as the biggest of my biggest mistakes, I’m getting ever closer to retirement and I’m at too much risk of a single-sector or single-stock collapse. I really must do something about it.

Alan Oscroft owns shares of Premier Oil. The Motley Fool UK has recommended Barclays. 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

A senior man and his wife holding hands walking up a hill on a footpath looking away from the camera at the view. The fishing village of Polperro is behind them.
Investing Articles

ISA or SIPP? Here’s 1 advantage and 1 disadvantage of both

SIPPs and Stocks and Shares ISAs both have potentially attractive features, as well as downsides. Christopher Ruane looks at some…

Read more »

Portrait of pensive bearded senior looking on screen of laptop sitting at table with coffee cup.
Investing Articles

£1,000 invested in Lloyds shares 6 weeks ago is now worth…

Lloyds shares have been on a huge run in the last couple of years. But is a 15% pullback in…

Read more »

Man smiling and working on laptop
Investing Articles

After the FTSE 100’s slump, these bargain shares are calling!

Are you on the lookout for top cheap stocks to buy? Royston Wild reveals three FTSE 100 value shares he's…

Read more »

Man hanging in the balance over a log at seaside in Scotland
Investing Articles

Worried about a stock market crash? Here are 2 things you should know

A stock market crash may look plausible, but it’s far from a done deal. Still, if markets do wobble, I…

Read more »

piggy bank, searching with binoculars
Investing Articles

This FTSE 100 stock soared 900% — but after a 25% crash, is the rally over?

After blowing away the FTSE 100 in 2025, this miner has hit turbulence in 2026 — Andrew Mackie investigates what’s…

Read more »

A senior man and his wife holding hands walking up a hill on a footpath looking away from the camera at the view. The fishing village of Polperro is behind them.
Investing Articles

How much do I need in an ISA for a £700 second income?

Investing in dividend shares can be a great way to target a second income from a Stocks and Shares ISA.…

Read more »

Businessman with tablet, waiting at the train station platform
Investing Articles

If there’s a stock market crash this week, will you be ready?

Christopher Ruane explains why he's not phased by the inevitability of a stock market crash -- but is actively preparing…

Read more »

Mindful young woman breathing out with closed eyes, calming down in stressful situation, working on computer in modern kitchen.
Investing Articles

£15,000 invested in Diageo shares 3 weeks ago is now worth…

Bad times for Diageo shares! The last three weeks have seen yet another drop, but is this a time to…

Read more »