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I have the same issue with the guitar, and also plates at home. It's easy if it happens in the Sim home, as you can just drag the object into the Sim, putting it in his/her inventory, but the one that happens in festivals is impossible. I cannot drag it to the Sim inventory, I cannot right shift click on it, I cannot do anything about it!
It would be nice if we can actually modify the lot and so fix these bugs ourselves!
It's kinda baffling that the official MAXIS team couldn't reproduce this. All I can conclude is that the official MAXIS team didn't play the game as often as us, surprise surprise