A red flag serves as a warning.
An example of a red flag is when a series of numbers don't add up which could be because one of the numbers is listed incorrectly.
- a warning flag
- Informal something serving as a warning of trouble or danger ahead
- Informal something that arouses anger, irritation, revolt, etc.
- A warning signal.
- Something that demands attention or provokes an irritated reaction.
(plural red flags)
- green flag