Can Drug Test With Mouth Swab Produce Inaccurate Results?

I took a drug test with a mouth swab at a staffing agency for employment, this came up positive for THC.
This is IMPOSSIBLE! I DO NOT SMOKE POT EVER!!! So obviously this means this mouth swabs are not
100% accurate and cannot be used if they have false positives. Can you elaborate on this. Thank you. Angie