For your information, XX solid(s) were not loaded due to errors in the file. Would you like to resave your map without these solids?

How does this happen? Is there a way to find a out what couldnt be load or at least where that has been?
What is a solid anyway? a brush? an entity?

I have had this error once and just told the map to load without the solid, and I could never find what it didn't load.

I believe a solid is a brush that is tied to the world, as in a brush that isn't an entity but I could be wrong.

I have no idea what causes the error, possibly a brush that you were creating into an entity (such as a func_detail), but accidently deleted the classname before you saved and closed the map, so it had no classname to load with?

Hopefully someone else is more knowledgeable about this issue then me on here

If a solid can't be loaded that often means it's degenerate: vertices are placed in such a way that is it's a valid shape, and Hammer can't work with it. You can cause this by clipping, carving, or vertex editing. Generally those solids are tiny flecks left over when a larger shape was clipped imprecisely, and it's barely noticeable when they vanish.

|a..........b|
|.............|__
|.............c..................d|
|..................................|
|f__e|

Corner "c" is not allowed in a solid. The game unloaded the solid from the map to prevent any weird behavior. All you have to do is recreate the solid in question and divide it appropriately. Of course, sometimes the solid that is removed has no problems. Hammer is kind of weird like that. Annoying as this behavior is, you get used to it.

It runs though your level and fixes these brushes by putting all the vertex points on grid.