Eva Longoria's Restaurant Beso Sued -- Worst. Birthday. Ever.

Eva Longoria's famous Beso restaurant in L.A. is the slickest place in town ... so slick in fact, that some woman claims she slipped and fell on her ass there at her own birthday dinner.

According to a lawsuit filed in L.A. County Superior Court, Luiza Boyadjian claims she went to Eva's eatery last year to "celebrate her birthday with friends and family."

Boyadjian claims she tried to get up from her chair and walk around, when she "suddenly slipped and fell due to the floor ... being extremely slippery."

Boyadjian claims she's suffered major physical injuries as a result of the fall -- including damage to her nervous system.

Boyadjian alleges the restaurant had a duty to maintain safer floors -- and she's suing for unspecified damages greater than $25,000.