Ghost Brushes are formed usually when vertices are off the integer grid. They are indicated in-game by two things:
This happens when one or more vertices are along a decimal grid, rather than a integer grid. This is fixed by ensuring that all brushes are along the integer grid. You may need to go through extensive checking to ensure that all vertices are lined up on the grid.
One possible “quick-fix” is to set the grid size to 1, perform a “select all”, then “align to grid”(usually, Ctrl-G). This may or may not resolve the issue, but it is a good starting point. Re-compile your map, and see if the ghost brush remains.
Sometimes when vertices have been moved a lot, or even just a little, they slip off of the integer grid, and thus create ghost brushes. This happens even if you have an integer set on gridding. Because of how vertices are connected to other vertices, moving one can inadvertently move others as well, resulting in a configuration that does not line up on the integer grid. If after trying the “align to grid”, your ghost brush remains, try deleting the offending brush and recreate it, ensuring that it is on the integer grid.
If you need a custom shape, often it is better to use the Clipper tool(shortcut key : X), as it will only give you cutting points that are aligned to the grid size you have currently selected. You should never get a ghost brush by using the clipper tool.
However, if you insist on editing vertices by hand, just remember that moving one vertex affects the others it is connected to, and that they all must form a single flat plane. This requirement is the reason for the necessity of splitting lumps into 2 parts when building a sloped curve.
It is note-worthy that 1.5.0 has fixed this problem by changing the variable types to “floats” as opposed to “integers”.