Deleting a subgroup - getting an error message

I'm experiencing the following issue -

I created a new Group and I wanted members of a specific Team to also be included in that Group - I wasn't able to select their Team, so I hastily added the Group those people are related to instead. That created a SUBGROUP relationship I did not want. (Why is there no "undo" button??) I went to copy that subgroup so it was back to it's own "master" group, and wanted to delete the subgroup that had been created.

When I delete the subgroup, I get the following error message: Database error due to constraint violation.

What does this mean? How can I remove that subgroup I created in error?