What you've got there is a mismatched brush... the black line is actually the edge where one block meets another. In some cases this is unavoidable... the engine just can't join walls properly at certain angles or if there are complex brushes.
The easiest thing to do though is to zoom all the way in on your 2D windows and see if the brushes are perfectly aligned. If not, fix them!
Everyone and their mother says not to turn off the grid in the editor, but I've foind that the only way I can get walls lined up perfectly is to turn off the grid and do it by eyeball at the highest zoom.
When all else fails, add another brush to cover the crack!
:8: Captain Gillette
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Download CTF-Paranoimia!
The easiest thing to do though is to zoom all the way in on your 2D windows and see if the brushes are perfectly aligned. If not, fix them!
Everyone and their mother says not to turn off the grid in the editor, but I've foind that the only way I can get walls lined up perfectly is to turn off the grid and do it by eyeball at the highest zoom.
When all else fails, add another brush to cover the crack!
:8: Captain Gillette
=========================
Download CTF-Paranoimia!
but the benefits of being able to get things to fit properly far outweigh the problems with using it.