Normally, we'd call this a blooper -- an outtake from the set of her rom-com "Anyone But You," which was filmed in Australia -- but it's hard to laugh when you see, and hear, her scream.

Syd described it as a "soul scream" when she revealed the clip on Wednesday night on 'The Tonight Show.' She says she was supposed to act as if the trained Huntsman spider bit her, but then it truly had a feisty nibble on her hand.

As you can see, her eyes are wide open in panic ... and she shrieks at the top of her lungs while convincing others, including costar Glen Powell, that she's actually being bitten -- and not faking for the scene.

Huntsman spiders are widely lurking around Down Under, and while their bites are painful, their venom doesn't harm humans. Docs on set immediately checked out Sydney and she was fine.