Some times it just happens. I keep all my boxes in the low to mid 60 range, and haven't had any for years, until recently. Had an unwrapped LFD Limitado V sitting under a bunch of unwrapped assorted DE Ligas in my premium box. I went digging through that box one day, and the LFD had so much mold in the foot that it looked like it was wearing a powdered wig. It was so much that I wasn't willing to take a chance, and had to toss it. The unwrapped Ligas sitting right on top and along side of it, never picked up any mold whatsoever. Sometimes it happens from over humidification, and sometimes it just happens, even at lower humidity levels.