Using [ctrl Z] will sometimes create this error so don't use it. Perhaps you can find a better description in the forums.

At the top of the page click on FORUMS
Then scroll down to COD Mapping(not UO) and click on it.
Then click on new topic and on the next page under where it says 'Please Search Before Post' type in duplicate plane and click on search, read through the posts maybe something there will explain it better. I used COD instead of CODUO because the discussion is more extensive.

I'll give it a shot and see what happens ..
Thanks for all your help it is much appreciated ...
I have no idea what I am doing.

I have read through all these posts and if I can add my penny worth:
Shadows for trees – You are asking a model (entity) to cast a shadow. I had the same problem some time back and my fix was to click on the Flare option button in the CODUO maker and check the
-ModelShadow box. In doing so I got the shadows I was looking for.

As for the other issues I would seriously take on board the other suggestions to fix all the bugs that you have. Even if it means copying buildings, bridges and whatever you have made and place each one of them into a new map separately with only the bare minimum required to run it as a map; i.e. one spawn point and one intermission. Make sure you check the multiplayer option in the map properties. That way you can compile each map and sort out the bugs. You can then save the building etc. as a prefab knowing it is bug free and place it into your main map. Above all what ever you do keep checking by compiling and testing your map every step of the way because any cock-ups can be fixed easily before you get in too deep.

Good advice necrophaige, always compile your map as you go. That way you can eliminate problems as the crop up. Patience is the one virtue mappers can not map without. I'm sure you'll get it worked out.

Thanks for all the advice!! got my head swimming and so many ideas not sure where to go.. These settings that you speak of where are they located.? Are you using a program to compile.? If so what is it.?

Also Gomer you spoke of the entity being to big and needing to be cut down how do I do this ??

I have lots of patience and not in any hurry to throw out a map not completed.. I want to learn how to do this the best way possible..

I have started moving the map to a new map and fixing things on the way. Hopefully this will work and it will be easy to do ;0) So far it is working well.. Thanks for the Idea.

The utility I use for compiling maps can be found here:

http://modsonline.com/Downloads-full-2623.html

Never had any problems with it.
Your project may seem daunting but just take it one step at a time. Search the forums and tutorials for things you are unsure of as they often reveal the simplest of solutions to what appears to be the most complex of problems.