Warren Buffett once called Tesco (LSE:TSCO) a “huge mistake”, admitting that “dawdling” before selling cost Berkshire Hathaway $444m. It was a rare and painful admission from one of history’s most successful investors.

As a shareholder, I think the most useful part of Buffett’s mistake isn’t the loss itself. It’s what it teaches investors about business quality, management and timing. Here are my three key takeaways.

Market dominance comes and goes

As the UK’s leading grocer, Tesco was just the sort of business Buffett liked and understood: a huge, cash-generative company with a powerful position in everyday spending. He built Berkshire’s stake in the grocer to around 415m shares by 2012. 

But by 2014, Buffett wrote that Tesco’s market share was falling, its margins were contracting and accounting problems had surfaced.

A possible misjudgement here was assuming that scale and dominance alone was sufficient to keep competitors at bay.

That still matters today. Competition from discounters, online rivals and shifting consumer habits mean Tesco must keep innovating to stay ahead of the curve. Its scale, loyalty card strength, and own-brand range are key advantages but they require constant investment.

A moat isn’t a wall you build once – it requires constant maintenance and upkeep as the environment changes around it.

When the thesis changes, act

Buffett said he began to “sour” on Tesco in 2013 and sold 114m shares, but held the rest for too long. In his 2014 shareholder letter, he wrote: “An attentive investor… would have sold Tesco shares earlier”.

That’s the lesson many private investors ignore. A falling share price is uncomfortable, but it isn’t a reason to ignore deteriorating fundamentals.

An investor could use a simple rule: if management credibility, cash generation or competitive position breaks, revisit the holding straight away.

That doesn’t mean selling on every bad headline – simply, reassessing whether the original reason for owning the shares still exists. A good investor always chooses evidence over emotion when making a decision.

A company can recover from disaster

Buffett sold Tesco after the accounting scandal and profit warnings, exiting at a loss. It was a dark time for the supermarket, but it’s not the same business today.

For example, its latest interim results show impressive growth:

Those numbers don’t negate the past, but they do show that a quality company can bounce back from one weak period or singular mistake. Tesco has clearly done a solid job of rebuilding trust, strengthening its numbers, and rewarding its shareholders.

What I take from it

Buffett’s Tesco saga teaches three things:

• Even the best businesses face hiccups now and again.
• When the narrative changes, take action and be decisive.
• A quality business with good management can recover.

I continue to hold Tesco today because the business is generating cash, returning capital and rebuilding the credibility it lost. Of course, the road ahead could still face challenges, but it’s proven it can handle them.

That’s the kind of stock worth considering for a long-term retirement portoflio.

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Mark Hartley owns shares in Tesco.