Yes, window boxes can potentially cause damage if not installed and maintained correctly. Poorly sealed window boxes may leak water or insecticides that seep into your walls and cause long-term structural damage. If a window box is too heavy and placed on an ill-prepared wall, the weight of the soil can put stress on it and eventually lead to a collapse. To prevent any damage, make sure to securely attach a window box to your house with proper fasteners, use quality liners that won't rot or crack over time, and keep the weight of the filled soil within recommended limits.