So, When I first started stardew valley I thought. "awe this cute little cave will be my perserves area" Then one day Demetrius shows up asking if I wanted fruit or mushrooms... Picking mushrooms results in loss of whatever is in the 6 cells taken up by the newly acquired mushroom boxes. There is no way to get around it due to having just walked out your door from a good nights rest (save). He should let you clear it out first.