Man Smacks Server With Plate at London Restaurant on Video

Turns out the customer's not always right ... sometimes they do stuff that's straight-up wrong -- like smacking their server in the face with dinnerware!

New surveillance video from a Nando's restaurant -- a popular fast-food chain in the UK -- captured the viral moment ... showing a man in a hoodie and beanie grabbing a dinner plate and smacking a waitress across the face when she puts the family of 3's dinner on the table.

The server's clearly stunned after taking the dish to the face, then runs off ... while the man, woman and child at the table quickly exit the restaurant.

It's not clear what motivated the vicious plate slap ... though a woman claiming to be the stricken server in the vid later posted about the incident -- she says the cops totally screwed up their handling of it.

The woman, whose account says her name's Zahra, claims cops were in the restaurant when this all went down ... and, she claims they did nothing to help -- they didn't even check CCTV to review the incident, she says.

MailOnline received a statement from Metropolitan Police admitting the attack was reported at the time of the March incident, and confirmed officers didn't follow up as they should have. An investigation is ongoing.

Seems this offender served himself up on a silver platter at the time ... but so far, no arrests have been made.