Yea somehow I ended up making an invisible wall in forge and have no idea how to get rid of it. Basically I was in forgeworld in that big cave area and lined up 2 coliseum windows and the left one disappeared, so I thought "WTF? Well whatever I'll just put down another and move them back a bit closer to the tanks." After I lined it up I walked up to it to see how it looked close up and I couldn't walk any farther where I had placed the magically disappearing wall. I didn't see any threads when I searched on here so I don't think it's been asked before.
LightsOut225 ran into this problem a looong time ago with Rock Arches. I can't recall if he fixed it or not, but I'd ask him about it next time he's online.
I've ran into this problem twice now while using brace larges. Odd enough, with two different, equally bad problems. 1. I deleted all brace larges and the problem was solved. The down side is that I, obviously, had to redo everything previous to that. 2. I deleted all brace larges and the "invisible wall" remained. I have absolutely no clue how to fix this. How far are you into your project? Is is too late to just delete all coliseum windows?
How far is "too far"? Like 3/4ths of the way in? I'm cooking up some interesting ideas for this odd glitch I've never encountered before.
It is repeatable so if you do the same thing again you will still have the problem. To remove the object, give all others a label that does not affect anything then spawn a new one and hit delete all. The labels prevent the other objects of the same type from being deleted. Sometimes by changing the coordinates slightly the problem will go away so before you delete the glitches object be sure to hold the object you're using for the delete all in the exact same place so you can record the coordinates. If you want more info on this, just search the forum. Your not the first one to have this problem.
If you have any coliseum walls on the map, it's probably those since they like to turn invisible randomly.
I believe that I had the same problem as you not to long ago. I lined up four coliseum windows in the cave, and one of them would keep disappearing if it was approached and it could not be picked up or deleted. If you don't want to delete every Coliseum window on the map you have to select a gametype that is not Basic Editing and label the coliseum windows that you do not want to remove with a gametype label, and then spawn another coliseum window and use the "Delete all" option. If that doesn't work, you will probably have rebuild the entire map.