Worst-case scenario: the UEd Goblin wipes the map and burns down your house.
Legacy:Hall Of Mirrors
What is a Hall of Mirrors (HOM)?
The Hall of Mirrors is the scourge of 3D level design. A HOM is a common mistake/error where there is a surface that seems to mirror objects in front of them (this is not entirely accurate, but this is how the HOM got its name), or create visual copies of them.
HOMs are not quite the same as a BSP Hole, though a BSP hole is often the cause of a HOM.
What causes a Hall of Mirrors effect?
A hall of mirrors effect is usually created when a objects are drawn to the screen without erasing the previous frame. The most common reason for this occurring in the Unreal Engine is a BSP hole, a transparent/translucent texture on a wall that has nothing behind it, or too short distance fog on a map with no SkyBox.
How does this create this effect?
When a frame is drawn by the game engine, it is usually drawn over the previous frame. When there is a HOM effect, there is nothing to cover up the previous frame, so anything that moves in front of the surface of the HOM will "imprint" on the HOM. If you use the ghost cheat, the HOM will appear black because the ghost cheat has the additional effect of drawing a black square behind all objects on the screen, for the purpose of preventing HOM effects when inside a wall.
What does one look like?
An example using Half-Life can be viewed with the link below:
Sometimes this is fixed with a rebuild, sometimes not. One effective technique is to add another brush in such a way that it is flush with the surface.
The best way is to study zone/portal view to work out which surface might be responsible. Reordering brushes or changing the way an area is built may help.
How to Cause the HOM Effect (deliberately)
If for some reason, you WANT to introduce a hall of mirrors effect, you can do so by setting a surface's 'invisible' flag to true. This, of course, only causes a HOM effect on surfaces that "border" the world, like those that would normally be designated fake backdrop.
NOTE -> HOM's Are VERY VERY Bad, and Ugly !
Mosquito: Is there a page that lists common mapping errors? Errors such as the HOM effect and BSP holes and possible textures not working, I don't know any others. If there isn't I think we should make one. A page like that can be very helpful for newbie mappers that have a similar problem and can't figure out what to do. Pages like the BSP hole are very helpful for someone that doesn't know what to do. I think we need to cover more problems in such detail as that.
GRAF1K: I'll bet there's a few little-know HOM fixes floating around. Here's a good place to post them.
MasterOfTheDark: I made a relatively simple (bad for very large levels, but hey, it worked) fix for the HOM effect: I subracted a hollow box from around a room that caused the HOM effect and made a tiny hole between the two. You can find my sample here (22 KB): http://motd.4d2.org/UT/fixHOM.ut2 A skybox might also fix the problem, though I haven't tried.
GRAF1K: The 1on1Pack has used your HOM fix, see DM-1on1-Tangora.
T1: Merged info from HOM to here.