Jim Reid, macroeconomic research guru at Deutsche Bank, has issued a warning that sets off alarm bells: investors are borrowing money at a frantic pace to buy more stocks, a behavior similar to that experienced in the wake of major crises. This is called "margin debt," which is when an investor uses their own stock as collateral to borrow money and continue buying.

This is reflected in the data from the New York Stock Exchange, which reveals that between May and June, the fifth-largest month-over-month increase was recorded since 1998. And here's the worrying part: the only times this indicator rose more were just before the dot-com bubble burst in 2000 and the financial crisis of 2008.

Is it possible to see a bubble burst again?

Reid acknowledges that, although the current surge hasn't reached the extremes seen then, we are witnessing one of the most aggressive rebounds in recent years. But there is another worrying fact: if we compare this debt with the size of the global economy, it already exceeds the levels of those years of unsustainable euphoria.

The warning comes at a time when the S&P 500 has recovered more than 30% from its April lows, when Donald Trump threatened to raise tariffs, and last week hit all-time highs again. However, it's not all optimism. The emergence of so-called meme stocks—such as Opendoor, GoPro, and Krispy Kreme—driven by retail traders on forums like Reddit, has fueled speculation. These investors are betting against large funds, causing sharp increases in highly shorted stocks. Are we repeating the mistakes of the past? The numbers suggest we should at least keep our eyes wide open…