Databases are designed to support multiple users. This presents a potential problem for development teams when multiple developers may be making changes to the database. When you open a database object such as a table in a designer window, Visual Studio .NET does not lock it for exclusive access. Two users may therefore have the same object open for editing simultaneously. This will lead to a problem when the second user tries to save any changes.
Visual Studio .NET deals with this scenario by refusing to save the second user's changes. If it detects that the database object has been modified since editing began, it will not make any changes. The second user will therefore be obliged to reapply the changes. You should therefore save early and save often.