
Rueful
Slightly Humorous
Rueful can express regret with a touch of irony or wry amusement, showing one can also see humor in their own faults. 
Subtle Regret
It's used for mild regrets, not deep sorrow or guilt; appropriate for trivial mistakes or minor offenses. 
Facial Expressions
Often associated with specific expressions, like a rueful smile, indicating an acknowledgment of a misstep. 