They say the customer is always right, but not at Karen's Diner apparently.

Unaware of the restaurant's infamously awful way of dealing with diners, one gran threatened to ‘knock out’ a staff member in a now-viral video.

Even more hilariously, staff struggled to stay in character as they served the miffed matriarch.

The hilarious incident happened at the Sheffield branch, with the pensioner completely unaware of the taunting that was about to be dished out.

For those foodies who haven’t visited yet, Karen's Diner is notorious for being rude to its guests and hires actors to serve up various insults.

This makes it all the more hilarious as the grandmother enters the restaurant and is shocked to hear a staff member swearing.

"Excuse me, watch your language," she says, telling off the waiter as if they were one of her own grandkids.

Tiktok/elodystarx

And in a very on-brand manner, the server responds: "What you gonna do about it? Shut the f*** up."

While most of us might have realised the theme by this point and taken the insult with a pinch of salt, the pensioner refuses to back down.

“If I were younger I'd knock you [out]” she furiously tells them, as the staff member and the gran get into a full-blown bun fight.

The now-viral video gets even funnier as a male colleague begins to taunt the table, with the elderly woman telling her loved ones: "Who's picked this bloody place?"

Thankfully, the gran was eventually realised it was a novelty diner, as the waitress added: "Welcome to Karen's Diner. Come on you silly old b***h."

In fact, in follow-up videos the grandmother is seen joining in with staff and getting a round of applause from customers, as she parades through the various booths.

Unsurprisingly, the video has gone viral after being uploaded to TikTok by the woman’s granddaughter, Elody Star, who explained in a follow-up video to her followers that her gran 'enjoyed her experience'.

"[She] knew what was happening after we let her know after we sat down shortly after arriving, she even took a photo and apologised for her bad language to the staff while leaving!"

Tiktok/elodystarx

Commenters also can’t seem to get enough of the punchy pensioner, with one writing: “Granny was ready to take on all the staff for the whole family.”

And another added: “I honestly don't understand how it doesn't escalate into a fight.”

Meanwhile, others were surprised by the later sweet moment between the grandmother and staff.

A third gushed: “awh the lady was actually sweet and realised Nana was actually getting upset so let her in on it.”

Former restaurant staff also reacted to the video, saying that they wishes they could have treated customers like the Karen’s Diner staff do.

“As someone that spent many years working in restaurants, I would have given my arms to be able to speak to some customers like this,” joked one server.

It seems like Karen's Diner will continue to make a meal of things, if this is the reaction it gets.