Problem: The add field and delete field commands are enabled on read-only tables in ArcMap
This problem is addressed in ArcGIS 8.1 Service Pack 1. See the Related Information section to download this pack.
The table window in ArcMap has a delete field command in the column context menu and an add field command in the options menu. If the table is from a read-only data source (i.e., OLE DB connection, delimited text file), these commands should be grayed out. However, if you are working in the Show selected mode in the table window, these commands are enabled.
If you try to add or delete a field, it will appear that the command completed successfully. However, the underlying data will be unchanged.
If you are using layers based on XY or route events, this may cause ArcMap to crash.
Solution or Workaround
Avoid using these commands with read-only data. These commands will be grayed out in Show all mode in the table window if the data is read-only. You can use this to determine if your data allows fields to be added or deleted in ArcMap.
If you need to add a field to or delete a field from one of these data sources, do it outside ArcGIS. Use a text editor for text files or RDBMS tools for OLD DB data.