i saw Hye Jin holding the puzzle piece that Seung Joo gave her back when they were little, but didnt Ha Ri give him the puzzle piece a couple episodes back? or did i just miss something..
I thought exactly the same thing.Maybe I did miss something as well but I think she might have just showed him the puzzle piece and not given it to him and kept it.
I remember a few episodes back, after Ha Ri gave Seung Joon the puzzle piece, he placed the missing puzzle piece back onto the puzzle and looked at it for a few seconds. I do not remember exactly, or was I hallucinating lol.
He didn't place it back in the whole puzzle. He left it as is even after seeing the puzzle piece from Ha Ri.
I just assumed Ha Ri made a copy so that Hye Jin wouldn't notice it was gone. 
I long considered it to be the loophole of the drama. How does Hye Jin not realize it's gone all of a sudden, considering it was so important to her and she was frantic about losing it earlier in the office?  Then Ha Ri makes no attempt to return it back in its place (on HJ's desk) after she showed Sung Joon she "still had" the puzzle piece.