You wanted “not common”, sounds like. “Not a thing” is a confusing idiom. It specifically means never happens, ever, but the idiom is frequently used in contex

You wanted “not common”, sounds like. “Not a thing” is a confusing idiom. It specifically means never happens, ever, but the idiom is frequently used in contexts where hyperbole is expected, so a literal translation accounting for the hyperbole might be something more like “I’ve seen it once, but it’s vanishingly rare”.