When I increase the quantity of workspaces to 7, the right margin appears between the right edge of the screen and the workspaces list in the overview mode (see the picture*).

1) Set the number of workspaces (NOW) to six;

2) Go to the overview mode -- there won't be the right margin;

3) Increase the NOW by 1 -- the right margin of 9px will appear between the right edge of the screen and the list of workspaces.

NOTE: I assume that the actual number of workspaces needed the bug to appear can differ on different installations.

Arch Linux; GNOME 3.10 -- 3.12.
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* https://dl.dropboxusercontent.com/u/20481401/archlinux/issue_with_workspaces.png