Troubleshooting: Why does my trace return results from multiple networks?
You may have a map document containing data from two personal geodatabases (e.g., gas data and electric data). If you trace the electric network and opt for the results to be selected, the resulting selection set will include both gas and electric features.
This is a known Esri issue.
The work-around is to put the classes you are tracing under the other network's classes in the Table of Contents. Thus, if you were tracing on gas, you'd have your electric layers on top of your gas layers. Another option would be to use set Selectable Layers such that you only select the layers you want.