New Delhi, Nov. 4 -- On 10 June 2000, London opened the Millennium Bridge. Around 90,000 people crossed it that day, with 2,000 on the deck at once.

It began to sway sideways.

Each person adjusted their steps to stay balanced. But everyone adjusted in the same way, at the same moment. Those corrections pushed the bridge further, prompting even more people to correct.

The bridge closed after two days. Engineers found that just 160 walkers could set it off. The bridge was well built, but what made it unusable was that thousands of individually reasonable decisions, taken together, added up to an unreasonable outcome.

I think about that bridge whenever I see investors buying what has already done well. When everyone does it together, it ...