I think both times it happened, I was adding created objects or cad details I had modified back to the library as the same name. For instance this last time I modified a footing detail I have and added it back to the library. I got a warning about the block having the same name. (See attached) I think I selected yes to replace the items. I had two different blocks with the same name. The new one was fine but the old one, I couldn't move, delete, rename, or export without getting an error message.
I tried to duplicate what I thought was the problem this morning, but didn't get the same result.