Complex edge features with an invalid geometry error 12, identical start-stop vertex, cannot be edited while simple edge feature can be edited. The junction at the start-stop vertex can be moved, which can result in incorrect connectivity.
Other features can be snapped along the length of the edge. The feature is split, creating two valid network features from the original invalid feature.
This is a known issue.
Depending on the version of ArcGIS, the help file may be in a different location:
For ArcGIS 8.3 and earlier:
Follow the steps for 'Repairing network topology' outlined in the ArcGIS Desktop Help under Geodatabases > Editing your geodatabase.
For ArcGIS 9.0 and later:
Follow the steps for 'Repairing network connectivity' outlined in the ArcGIS Desktop Help under Editing in ArcMap > Editing geometric networks.