i dont think anybody can have suffered and solved more problems than me (download my first map and you'll see why) with brushes going straight through another and out into the void. making a window look out into 'the void'
y'know, all stuff that resolves in problems.
the most common and most annoying to fix when it doesnt show up in the problems screen (shift + p) is perpendicular faces. god that's annoying.
anyways, long story short, all these i got fixed in 4 simple steps.
1. check compile log and mark down any errors it displays (if the compile only takes a second to complete, you have an error)
2. search TWHL forums for a thread already created about this problem.
3. if you find none, post compile log in a new thread and clearly state what you think the problem is.
4. Listen to teh monkehs