When I have a current selection and want to add another record to it, I sometimes use the Find/Select dialog and search for the record to add to the current selection. If I double click on a single record that is not part of the current selection, I get a dialog that asks if I want to add that record. If I say Yes, sometimes that record gets added, sometimes another different record gets added.
I believe this may only be happening on a Shared Database. Has anyone else seen this? It happens to me regularly and consistently with a particular need but not consistently all the time.