FindPortalSide: Couldn't find a good match for which brush to assign to a portal
Look farmiliar? This error can be from an array of things, but most of them have an easy solution on the net. I got this error when trying to compile my map so I loaded up a pointfile only to find the pointfile points at some random direction and never enters my BSP level. Perplexing. After a little bit of googling I fixed it.
Go to edit > select all, and then tools > center origins.
The reason this exists is because all the brush entities and world geometry brushes have 'origins', the place that they were created. Sometimes after some editing, you may move the brush but the origin point stays the same. If the origin point is outside the map it will leak, but you'll never be able to see why. Try the above solution.