Today, if I open any article or category (but not talk page), such as https://en.wikipedia.org/wiki/Great, the "Edit" and "Edit source" tabs both have "<undefined>" added to the wording. It appears to be a separate blue link to the same destination. It looks strange.
More seriously, if I have the window set to a width of about 80% of my laptop screen or less (which I usually do as I find it easier to read and enables access to some of the desktop icons and gadgets such as clock and calendar), then the "Read", "Edit <Undefined>" and "Edit source <undefined>" tabs move constantly from side to side in a desperate attempt to display greater width than there is room for. The remaining tabs and the search box jump up and down, and the whole effect is very distracting.
I'm using Firefox and WIndows Vista.