Melissa McCarthy may let obnoxious and ignorant comments about her weight roll off her back, but she's not so willing to turn the other cheek when she sees a child being mistreated.

TMZ reports that McCarthy threw an extra off the Wilmington, N.C. set of her upcoming directorial debut, 'Tammy,' when she witnessed a fledgling starlet getting rough with her own son.

The woman in question -- a 20-something with a child who's 4 or 5 years old -- was a $58-per-day extra on hand for the day-long shoot.

Witnesses say that she'd been struggling with her son all day, loudly telling him to keep quiet and "stop it," though it's unclear exactly what the kid was doing to be such a pain. (Probably, you know, being a kid.)

More disruptive than the child was his mom, whose constant nagging and harping was loud and frequent enough to actually disrupt filming -- filming that involved over 200 other extras, who maintained professional courtesy and ostensibly didn't bring any rugrats to work with them.

The final straw for McCarthy was when she witnessed the young woman grab her child by the wrist and yank him up in the air, causing the 'Identity Thief' star and mom of two to go into full-on Mama Bear mode.

She immediately had production assistants give the woman and her son the boot, telling them that she wouldn't tolerate that kind of nonsense on her set.

Moral of the story? If you're gonna get rough with your kid, don't do it in front of Melissa McCarthy. (Or, preferably, at all.)