red flag
Pronunciation:
noun
Definitions
A cue, warning, or alert; a sign or signal that something is wrong.
Example: Improper formatting is a red flag to look for other problems with the data.
In the United Kingdom and some other countries, a flag formerly used by a railway guard to warn the driver not to start the train.
A common symbol, usually in the form of a banner, for leftist revolutionary ideologies, especially socialism.