21.

“Standing in line at a retail store and someone carrying on a conversation with the cashier after they’ve checked out — basically holding up the next customer(s).”

—u/tiraralabasura_2055

“As someone who works in a bar, this annoys us too. Like if I have no customers, I’ll appreciate the conversation to cure my boredom. If I’m pouring your drink, then feel free to chat with me; it makes it less awkward. But if I’ve charged you, you have your drink in hand, and the bar is four deep, please just leave.”

—u/tocla1

Source: https://www.buzzfeed.com/ravenishak/minor-inconveniences-reddit