Here's a Civil 3D riddle for you. This one fell under the category of things that make me go hmmmmm. I was trying to add a subassembly and kept getting this error:
Then I was getting a weird ghosted image of a subassembly that was not properly aligned with the assembly base point.
When I tried to erase the subassembly, it wouldn't erase. It was like it was there - but not there; some kind of subassembly limbo. However, I could attempt to attach another subassembly to it and Civil 3D would recognize its existence but give me this error:
I eventually discovered the answer, but for fun I want to see if anyone else knows. Comment if you know what's going on.
Answer: The current layer is locked - there you have it!