A sticky residue on the carpet.
If the sticky residue is not removed, after a certain amount of time the stain will reappear since dust and dirt will stick to the residue.
First try cleaning with plain water:
Wet a cotton towel with plain cold water, wring it out and wipe the stain with wide sweeping strokes. Do not dab! Whilst drying cover the spot with a moist cotton towel.
If the treatment with water is not satisfactory, the sticky residue is most probably caused by some oil or fatty product. Fat and oil stains can be removed by using James Stainspray.
Spray James Stainspray on a cotton cloth and wipe the stain with wide sweeping strokes. Do not dab! Repeat treatment until the stain does not give off anymore dirt.