A “red flag” is a metaphor for a problem. A red flag is also a symbol of Communism. A joke was posted on reddit on December 4, 2014:
   
We should’ve known about the failure of communism
In retrospect, there were a lot of red flags…
   
   
Wikipedia: Red flag (politics)
In politics, a red flag is a symbol of Socialism, Communism, and left-wing politics; it has been associated with left-wing politics since the French Revolution (1789–99). Socialists adopted the symbol during the Revolutions of 1848 and it became a symbol of communism as a result of its use by the Paris Commune of 1871. The flags of several communist states, including China, Vietnam and the Soviet Union, are explicitly based on the original red flag. The red flag is also used as a symbol by some democratic socialists and social democrats, for example the Avami National Party of Pakistan, French Socialist Party and the Social Democratic Party of Germany. The Labour Party in Britain used it until the late 1980s. It was the inspiration for the socialist anthem, The Red Flag.
 
Wikipedia: Red flag (idiom)
The term red flag could mean either a literal flag used for signaling or, as a metaphor, a sign of some particular problem requiring attention.
 
The earliest citation for “red flag” in the Oxford English Dictionary is from 1602 and shows that at that time the flag was used by military forces to indicate that they were preparing for battle.
   
reddit
We should’ve known about the failure of communism (self.Jokes)
submitted December 4, 2014 by Parov
In retrospect, there were a lot of red flags…
     
Twitter
Simply_Godzy™
‏@Godzygodzy
We should’ve known about the failure of communism: In retrospect, there were a lot of red flags…  submitted ... http://bit.ly/1wy4Xm1
8:56 AM - 4 Dec 2014
 
Twitter
Terrible Jokes
‏@doyouevenjoke
We should’ve known about the failure of communism
In retrospect, there were a lot of red flags…
9:21 AM - 4 Dec 2014
   
Overheard at Harvard
“we should’ve known communism would fail. there were a lot of red flags.”
Dec 5th, 2014
 
reddit
We should have known Communism would fail. (self.Jokes)
submitted August 29, 2015 by StarBP
There were a lot of red flags.
 
reddit
We should’ve known about the failure of communism (self.Jokes)
submitted March 2, 2016 by TeamDodgy
In retrospect, there were a lot of red flags…