A warning shot is a global catastrophe that indirectly reduces existential risk by increasing concern about future catastrophes.
The expression warning sign is sometimes used to describe any event that increases concern about a particular category of existential risk, regardless of whether the event itself constitutes a global catastrophe. For example, plausible candidates for an AI warning sign include not only a catastrophic failure by an AI system but also public outreach campaigns or the publication of an exceptionally persuasive book on AI safety.
A related notion is that of a fire alarm, a warning sign that creates common knowledge that some technology—typically avanced artificial intelligence—actually poses an existential risk.
Note, however, that both "warning shot" and "fire alarm" are sometimes used as synonyms for "warning sign"....