Had an old Avalon Hill copy stacked flat under three other boxes in my closet for 2 years, the lid got crushed and cracked a corner. Lost about $80 of resale value, anyone else got storage horror stories from heavy stacking?
Stacking flat is the real killer, most people worry about sunlight or damp but gravity does the damage over time. Here's the thing nobody talks about, the weight doesn't even need to be heavy, it just needs to sit there for months and the cardboard slowly gives. I flip my boxes upright now so they hold their own weight on the edges instead of the lid.