EXCLUSIVE

Gives a whole new meaning to the phrase "burger joint" -- TMZ has learned, a Wendy's employee was arrested last month in Georgia ... after allegedly making a cheeseburger with one crazy off-the-menu ingredient ... marijuana.

According to the Lovejoy Police Department, 32-year-old Amy Seiber was busted during her shift on November 1 ... after a customer called 911, complaining she had found a half-smoked blunt in her cheeseburger.

The customer met with police at the Wendy's location where she ordered the burger, and cops claim Seiber immediately admitted the blunt was hers.

According to police, Seiber said she had been smoking pot on the job and conveniently "misplaced" the blunt inside the customer's burger ... right on top of the pickle.

The blunt was taken as evidence and Seiber was arrested for possession of marijuana ... oh yeah, she was also promptly fired.

We spoke to the lucky customer, who tells us she experienced food poisoning-type symptoms after the incident and had to be hospitalized.

So far, she says Wendy's has offered to help pay for her medical bills ... and has even generously thrown in a $50 gift certificate. Like she'll ever eat at Wendy's ever again.