Some well known ones
"Smile killer".
Or:
Please don't!

Sometimes sentences are formulated towards customers that are extremely negative and dismissive.
Customers react to this consciously, sometimes unconsciously, with rejection and disappointment.
No way are they smiling.
Here are some examples of very bad communication. The list could be expanded with many other incorrectly worded sentences.

"I can't tell you that."

"You're wrong with me."

"No, that's not possible!"

"I'm off work in five minutes!"

"I don't have time for you now."

"I can't help you there!"

"I don't know".

"What is it about?!"

"You have to do it now."

"Listen?"

"It can't have been like that!"

"Do you understand me now?"

"We always do it like this!"

"Where is the problem?"

"Now watch out, it works like this!"

"As a professional, I advise you!"

"Well, mistakes happen."