Is one who brings a pebble from the Jamarat, unknowingly, sinful, and what are the consequences in this case, and is there any truth to what is said, that whoever brings it will be afflicted with harm?

There is no sin upon you for keeping the pebble in your bag, and you may do with it as you wish. For none other than God possesses the power to benefit or harm another. Allah Almighty says: "And they worship other than Allah that which neither harms them nor benefits them." And whatever affliction befalls a person is by the decree and preordainment of Allah, as He Almighty says: "No disaster strikes except by permission of Allah."