3 investing mistakes I think you should avoid as the FTSE 100 slumps See all posts by Alan Oscroft Image source: Getty Images The FTSE 100 is bouncing around the 5,000 point level as government promises haven’t done much to settle nerves. But it’s hard to know, at this stage, what will help and what won’t — and the priority has to be health, not wealth.Many of us invested in shares are on short-term losses, on paper at least. But what should we do? We should, at least, avoid some key investing mistakes.5G is here – and shares of this ‘sleeping giant’ could be a great way for you to potentially profit!According to one leading industry firm, the 5G boom could create a global industry worth US$12.3 TRILLION out of thin air…And if you click here we’ll show you something that could be key to unlocking 5G’s full potential…Don’t listenAccording to investment professionals, it’s a good time to be in cash now. On one level, yes, that’s perhaps true. Or, at least, it would be a good time to be in cash had you moved to cash before the FTSE 100 crash. It’s not much use suggesting it now, and it could even persuade people to sell their shares and crystallise some significant losses.I’ve also seen so-called experts banging on about the importance of being diversified among various asset classes. They’re talking about bonds, cash, gold… Again, sure, being diversified away from stocks would have been beneficial had you done it before the virus arrived.But is diversifying away from stocks a good long-term strategy? If you have a decade-long horizon or more, I say no, because you’ll miss out on the best performing type of investment ever.So don’t make the investing mistake of listening to the experts and their wise-sounding hindsight.Avoid debtI did read one sensible bit of advice, and it concerns debt. With interest rates being low, a lot of companies are carrying high levels of debt these days. And in an economic crunch, those with big debts are the most at risk. So yes, it seems sensible advice to steer clear of companies carrying debt.But there’s nothing new here, and nothing that’s remotely specific to the current panic. I’ve been banging on about the dangers of debt for ages, and I’m especially scathing about companies borrowing big while paying top dividends. I think running a company like that is just asking for trouble. And as this little strand of RNA that’s causing so much mayhem shows, we just can’t tell when that trouble is going to knock at the door.I’ve recently highlighted Premier Oil and Tullow Oil as examples of how debt can potentially cripple a company. So I repeat my advice to not invest in companies with high debt. Not just for the coronavirus downturn, but for always.Investing mistakesTo quote that Kipling chap, this really is a time to “keep your head when all about you are losing theirs.”The case for investing in shares for the long term has not changed. All those companies are the only things out there that actually generate new wealth, while other forms of investment are really just shuffling wealth around. And I really can’t see how that can possibly change.Interestingly, over the past week or so when the Footsie has been sliding and dipping below 5,000 points, shares in investment services provider Hargreaves Lansdown have started picking up again. If people are panic-selling shares, those providing the services are making a small charge for every transaction. And that will carry on when the tide turns and everyone starts buying again.So I say stay calm, and don’t get swept away by the tide. 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