I wanted to make a nif that used the nif for the commoncrate02 and the nif for the milkbottle to make something that looked like a crate filled with bottles. I copied the branch for the bottle and pasted the branch into the crate nif and positioned it and saved it with a new name. When I duplicated an item in the CK and tried to change the nif to my new one it crashed the CK. What did I do wrong?

Please can you post a screenshot of the nif tree?

This -- or better yet a copy of the NIF that can be downloaded and looked at !

There are many things that it could be (ie. you say copied the branch -- which branch of what node in the nif - the entire NIF including the collision node or just the Trishape branch and where did you paste it, etc. ) - so much easier to look at it to see exactly what was done and what the problem might be rather than shooting in the dark at every possible interpretation.

http://tesalliance.org/forums/index.php?/tutorials/article/98-static-mashups-in-nifskope/, see if you followed the same process. Or, as everyone else is suggesting, please post a pic of your nif.