Caused by a brush entity such as a func_door or func_plat with no solid brushes, or a func_rotating with no origin brush. If you've added anything like that recently, check it, and add a solid brush if necessary. If you can't find anything, look though the entity list and see if you can find an entity that should have brushes but has somehow lost them. Note that a adding something like a clip brush is enough to make the entity qualify as having solid brushes, if you are using a model as a func_ entity.
This problem is easily solved using brush_cleanup from bobtoolz.
