A cue, warning, or alert; a sign or signal that something is wrong.
A signal that the race must be stopped immediately (and possibly restarted).
red rag
A common symbol, usually in the form of a banner, for leftist revolutionary ideologies, especially socialism.
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 bell rung to warn people of a danger.
A problematic or alarming aspect.