Flight attendant Jamila Hardwick dished to Inside Edition about what flight attendants want to tell you about flying.

One of the more shocking revelations: Don't order tea or coffee. The pipes for hot water on planes aren't regularly cleaned.

Flight attendants get annoyed when you order Diet Coke, too.

It's just too fizzy.

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Flight attendants know a lot about flying — and some of that inside knowledge is kind of gross.

Flight attendant Jamila Hardwick told Inside Edition one particularly alarming fact about what you should and shouldn't order when the drinks cart comes down the aisle.

"The thing about the coffee and tea, the pipes are rarely cleaned," Hardwick said.

Flight attendants have said the same to Business Insider. "Flight attendants will not drink hot water on the plane. They will not drink plain coffee, and they will not drink plain tea," one said in 2017.

Read more: 6 things flight attendants want to tell you but can't

Airlines are only required to disinfect their water tanks four times a year, Inside Edition reported. A new study from Hunter College even says most major airlines don't have water that passes federal regulations for "relatively safe, clean water." Another Environmental Protection Agency study says one in eight planes fails the agency's standards for water safety.

Scores of three or higher for the EPA's Aircraft Drinking Water Rule indicates the water is clean to drink. But the water on Spirit, JetBlue, United, American and Delta scored under a two in the Hunter College study.

Diet Coke isn't gross, but it will annoy the flight attendant

Hardwick also told Inside Edition that flight attendants just get annoyed when you order Diet Coke. For flight attendants who are hustling to serve passengers, pouring that drink just takes too long.

"I don't know what it is about the chemistry in Diet Coke and us being 40,000 feet in the air," she said. "By the time we pour it, we could have three other drinks poured."

Watch the full Inside Edition interview here.