I have the same issue. We started with 4.3, now working on 4.5, there’s a collision issue with 1 landscape out of 100, can’t delete and reimport it because it is part of world composition.
It has the same collision settings, same everything with all other landscapes, but the collision just doesn’t work when dragging dropping new meshes on it. Very frustrating.
This is not an actual fix, just a workaround in the editor. At run-time, depending upon the engine settings, collision still fails to work properly.
Still seeing this issue in UE 4.7.1. We were able to work around it when previewing in the editor by deleting all terrain and undoing, but this doesn’t seem to work for the packaged version of the map. The collision map is completely wrong for the geometry – our pawn seems to encounter invisible barriers where there is no geometry, and encounters slopes that fall through the terrain.
It does not happen when previewing through the editor after deleting the terrain and undoing. It happens consistently when playing as a “standalone game” or in the packaged version. If we set the LOD to Epic, it still happens, but portions of the collision mesh seem to be accurate. If we set the LOD to Low, Pawns just fall through the terrain without collision.
The collision on the terrain is “BlockAll”. We’ve tried every setting that would still apply to the Pawn without success. Our pawn is a round ball (based on the rolling starter).
I am not sure if that would help you, but I got a solution which worked for me.
Chose Scuplt tool from Landscape editor. And make the Brush size 50000000. Max you can do. Size doesn’t matter but just speeds things up.
Then lower Tool Strenght to 0.00000001. You will not see no change but this will force the landscape to rebuilt everything. Be warned that it will ask you to build lightning even nothing changed.
This is really a big problem. Surprising to see that there is no check or rebuild for landscape collision, but this worked for me at least. Hope this helps to you too. 
PS: This happens ALOT when you add extra landscape components to an exsisting one.
I’m using 4.7.2 and I’m getting this problem with my terrain, its quite frustrating if I really had to re create the terrains. Has anybody found new solution? I’ve tried all methods mentioned above, except recreate it…
Agreed. Rebuilding art assets isn’t really a sustainable solution to the problem.
I had this problem with 4.13 and may have fixed it
my terrain map was suddenly full of small invisible colliders
so i decided to remove the last few fbx mesh imports and the problem seems to have now gone away
not sure if the importing causes the problem but it has saved my terrain map from having to be restarted
so worth a try maybe
This literally saved my life. Thank you for posting this. I had a problem where my terrain suddenly was going nuts with collision thus I deleted the latest imported meshes and it solved the issue! Apparently, there’s a mesh with a huge horrible collision that was really throwing off my map. Thank you.
DOesn’t help here, darn ;0-0,nothing works, a few areas only but thats a few too many.Not workable.
OLD thread I know but there aren’t many that address this–bad math ?
The issue is still here, and I’m about to jumpship to Lumberyard- they have 32bit terrain anyway,whichis MUCH smoother–
