I'm not sure if this is the correct place to add this "question" or "bug report", but it might help someone else that receives this message.
When i try to register a table that is created within my MS SQL server (without arcgis) and the column GDB_GEOMATTR_DATA exists the tool "register with geodatabase" will fail with the error 999999.

With the help of MS SQL Profiler i found out that the column that already exists were GDB_GEOMATTR_DATA.
Preferably the geoprocessing tool would not try to add this column if it already exists, or at least inform the user what column that already exists?
Kind Regards
Andreas