A classic sign of a bubble is when you start getting unsophisticated (non-professional) investors that only see money and start chasing the successes of others. Two huge examples from the past 15 years:

1. Tech bubble. Everyone and their brother became day-traders, chasing the hot stocks and trying to make their fortune. People were throwing their retirement savings, spare cash, anything they had into tech companies trying to make a quick buck.

2. Housing bubble. Everyone and their brother became flippers, chasing the hot properties and trying to make their fortune. People were throwing their retirement savings, spare cash, anything they had into properties trying to make a quick buck.

Copy/paste intentional.