Examples of petty behavior

Refusing to acknowledge someone’s presence or ignoring them as a form of revenge.
Making passive-aggressive comments or giving backhanded compliments.
Exaggerating minor mistakes or errors made by others to make them appear worse than they are.
Seeking revenge for small slights or perceived wrongs, rather than attempting to resolve the issue constructively.
Refusing to help or lend a hand to others, even when it’s a small request or task.
Taking something personally that was not intended as an insult or attack.
Spreading rumors or gossip about someone to tarnish their reputation or image.
Competing over trivial things, such as who has the better phone or car, in an attempt to one-up each other.
Holding grudges over minor incidents or mistakes, rather than forgiving and moving on.
Criticizing someone’s appearance, clothing, or personal choices, rather than focusing on their character or actions.

Definition of Petty according to dictionary.com

of little or no importance or consequence: petty grievances.
of lesser or secondary importance, merit, etc.; minor: petty considerations.
having or showing narrow ideas, interests, etc.: petty minds.
mean or ungenerous in small or trifling things: a petty person.
showing or caused by meanness of spirit: a petty revenge.
of secondary rank, especially in relation to others of the same class or kind: petty states; a petty tyrant.