I have two parts in an assembly, neither part is referencing any other part.

I was editing part 1 and switched back to the assembly and did a rebuild to see how the changes affected clearance when I noticed part 2 was now broken.

I opened up part 2 and saw the image below. 

Random items in Tree have been renamed as blocks, and anything dependent on those items has been broken.

Anyone seen anything like this before?

Thanks.