How do I resolve issue with deleted table in SQL Server Database Diagrams?
This is a bug in Management Studio.

I was able to reproduce it three ways, on both 2008 R2 and 2012 tools:


Create a new diagram; create a new empty table (any name); delete the table; in the same diagram, try to create a new table with the same name.
Create a new diagram; create a new empty table; delete the table; create a new diagram in the same database, and while the first one is still open, create a new table with the same name.
Create a new diagram; create a new empty table; delete the table; create a new diagram in the same database; close the first diagram; in the second diagram, create a new table with the same name.


From this behaviour, I'm guessing it keeps a list of tables per database outside the context of a single database diagram, and when an empty table is deleted, the reference isn't being cleaned up properly. I believe the fix is to close all diagrams for that database, and re-open. I think it's lucky you were able to save the changes you made with no errors!

I would suggest reporting this on Connect if you feel it's really important. IMO, it seems unlikely it will be fixed because it's so minor, but you never know.