Even if you think your colleague or boss is completely wrong about something, you can counter with an open-ended question that shows respect and a can-do spirit.
The clerk, instead of rolling his eyes at the man next in line, could have asked himself why this woman was so bitterly complaining, after which one simple open-ended question might have opened up a fruitful conversation, not to mention the creation of a genuine human relationship, no matter how fleeting.