Two patrons at the bar are arguing, and it’s escalating toward violence by the time the party notices. The specific cause is up to the DM (a card game gone bad, a spilled drink, an old grudge, whatever fits the table). The details don’t matter much. What matters is the tension is rising fast and everyone else in the room is watching to see what happens.
Give the party an open set of options. They can ignore it and let the fight happen, back one side and make an ally or enemy out of whichever patron they didn’t support, or step in to de-escalate before it turns physical. However it plays out, the tavern owner remembers who caused or prevented the damage.
Personal note: I’ve had the original cause of the fight be between two rival adventurers arguing over who fired the fatal arrow into a now-dead monster. Additional premise idea might be a table of rough and rowdy customers tried to dine-and-dash but another table of more honorable customers tried to stop them.
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