It seems SourceTree wants to prevent you from closing the last open tab. None of these work:

• Right click -> Close Tab
• Middle mouse button
• Clicking the "X" in the upper right of the tab

This is not very intuitive, because the UI doesn't indicate that the action isn't available (the "Close Tab" item and the "X" button should be greyed out if this behavior is intended).

Curiosly, the last tab can be closed with Ctrl+W (only if it's not a "New tab" tab, but that seems like a different issue - 7109).