I'd say the most common one that always gets me is the 'betrayal that isn't a betrayal' twist. You know, the character who seems to be the ultimate antagonist, the one forcing the couple apart or creating the 'forbidden' element, is actually revealed to be orchestrating the whole situation for a secret, protective reason. It was all a test or a ruse to make the bond stronger.
Another one I see constantly is the hidden connection twist. The 'forbidden' nature of the relationship is built on a misunderstanding—they're not actually step-siblings because one was adopted, the rival families were manipulated into hating each other by a third party, or the dark secret that makes it wrong is based on fabricated evidence. It lets the author have all that delicious, angst-filled tension and then absolve the characters of any real moral transgression so they can have a guilt-free HEA.
The pregnancy reveal as a plot device is so overused it's almost a genre staple, especially in mafia or boss/employee stories. It often comes right as the external forces are at their peak, trying to tear them apart, and suddenly the stakes are 'forbidden love PLUS a baby'. It forces a reckoning, but sometimes it feels less like an organic twist and more like a narrative sledgehammer.
Finally, the power reversal twist is huge in dark romance. The submissive captive or the powerless ingenue is revealed to have been playing a long game, or possesses a hidden strength or identity that flips the entire dynamic. The captor becomes the captivated, the hunter becomes the hunted. It satisfies that craving for the underdog to ultimately gain control in a world built on imbalance.