Original list from dabble. Feel free to add to this.
Someone tells a major character a secret so explosive it changes their lives forever.
A minor character becomes more important than previously anticipated.
A major character is suddenly put in an unexpected and dangerous situation.
A random theft occurs.
Your main character attends a large gathering of complete strangers.
A relationship with someone important dies because of a misunderstanding.
A main character is offered a large amount of the local currency to do something that is morally ambiguous
A gift turns out to have hidden strings.
An unexpected kiss between the unlikeliest of characters.
A death causes all the characters to re-evaluate their purpose in life.
Fear forces a major character to take dangerous risks.
Half the characters are killed in an horrific accident.
A new love affair is discovered.
An old conflict is resolved in an unexpected way.
A valued friend turns out to have a hidden agenda.
A new character arrives. Every second thing they say is a lie.
An illness threatens to expose an old secret.
Your protagonist finds themselves in the place they least want to be.
A major character sees something they shouldn’t have.
A message arrives; unfortunately it was delivered to the wrong person.
A practical joke goes horribly wrong.