On Land

Environment Information
At Rill Architects we run ArchiCAD on Mac OS X. If you work at Rill, this is your stuff. If you don't, but you work in ArchiCAD, you may find something interesting. Anybody else, I don't know.
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Here is a most ancient and despised bug in Archicad's wall cleanup behavior.

In most cases, if two surface edges meet whose materials are the same, the line between the surfaces is eliminated.

Corner line
Where three walls meet, two of the walls will often form a corner, which results in a 'strong' line that will not be removed despite the matching wall surface materials.

Column in plan
Here's a workaround. Place a column element across the offending corner, in plane with the wall surfaces. The column's material should match the wall.

Walls cut
Because columns naturally subtract from walls (if their layer intersection codes match), the edges that had been forming the corner are removed.

Line fixed
The three surfaces of wall, column, and wall clean up normally.

Train wreck
Columns can also help heal complex 3D wall interactions. Here's a train wreck that no amount of reference line, layer intersection, or priority voodoo could fix.

Train wreck plan
Placing a column within the intersection...

Train wreck fixed
...fills the joint.

Depending on the situation, it might be better to show these columns in plan, or to place them on a 3D wall layer (A Wall3 in our case).

Angled intersection
Using a custom profile column, you can force cleanup of complex angled intersections in plan and 3D.

Angled intersection fill
Draw a fill over the intersection to create the profile.

Angled intersection profile
Within the profile, set the edges within the wall to the same pen as the wall's background color. Those edges won't clean up otherwise.

Angled intersection plan fixed
The profile column heals the intersection in plan...

Angled intersection 3D
...and in 3D.

Why we can fix these things but Graphisoft can't is a separate topic.