This happened when all the Hobbits fall down the hill and they see the mushrooms. Well, if you look at Frodos' hair you can see straw in his hair. But then when he says, "Get off the road!", you can clearly see that the straw is out of his hair. Then when they run down the hill and Merry says, "What was that thing?", you can again see that the straw is back in his hair.