Sammi 'Sweetheart' Giancola is coming back to "Jersey Shore" -- a plot twist which didn't just come as a shock to the fanbase, but to the cast itself too ... TMZ has learned.
Sources connected to the show tell us Pauly D, JWoww, Snooki, The Situation, Vinny and all the rest of the stars on 'Family Vacation' had no idea Sammi would be showing up to set this past week ... and that they were just as surprised as the rest of the world.
Of course ... the reactions were wild, or so say our sources. We're told it was truly a shocker for everyone involved -- and the raw, jubilant responses it evoked from people were wild ... laughs, tears and everything in between.
All that stuff was captured on camera, and we're told it will all play out on the second half of this season (the 6th, BTW), which is currently filming. No word on an air date.
Sammi's return is a HUGE deal ... seeing how it's been over a decade since she's been on anything "Jersey Shore"-related, and was the only OG cast member not to come back for 'Family Vacation.' The timing of her presence now, though, might make perfect sense.
Fact is, she's got a very rocky history with Ronnie Ortiz-Magro ... whom she dated on and off throughout the original run. Ronnie has since moved on, but the ups and downs of his life continue -- and he stepped away from 'JS' indefinitely 2 years ago.
The dude made a guest appearance last season, but he's not a regular cast member anymore -- and that could be why Sammi agreed to sign on and return.
Now that the whole gang is back together (sans Ronnie), we'll see what sorta shenanigans they get into. Sammi's always been a big personality and is not one to put up with s***.
Can't wait!