EXCLUSIVE

One of the best selling sex toys in the world nearly made porn star Missy Martinez the ultimate fire crotch -- according to her the thing shot flames at her lady bits, and now she's demanding answers.

Missy claims the fiery scene went down Thursday on set -- in the Valley, of course -- when she turned on her Hitachi magic wand vibrator, and the handle immediately heated up. She tells TMZ ... sparks flew out of the base where the cord connects to the device.

The really scary part is it happened inches from her moneymaker, and a vaginal piercing on her porno scene partner ... according to Missy.

She says she fired off a harsh letter to Hitachi, claiming the device is defective and potentially deadly.