
Leave somebody in the lurch
Unexpected Timing
This phrase often implies that the abandonment happens at the worst possible time for the person affected. 
Emotional Impact
Use this phrase to express a sense of betrayal or letdown by someone who fails to fulfill their promises or duties. 
Not for Emergencies
The phrase should not be used when someone can't help because of an emergency—it implies a lack of responsibility, not misfortune. 