The first time a new tab is created and closed the entire safari window fails to repaint and you are left with the window outline and a tab close button. Clicking that close button again causes the repaint and returns things to normal. Also, opening 3 tabs and clicking the close button next to the third one does not close it it just applies focus to the second one.
Might be related to bug 8577.
*** This bug has been marked as a duplicate of 8577 ***