See http://article.gmane.org/gmane.comp.documentfoundation.libreoffice.devel/9 It has been probably introduced by http://cgit.freedesktop.org/libreoffice/calc/commit/?id=e0bf78d6f572000cf5f9dda2a85dd15a8b350e18
This is my area.
This is fixed in the repo. When the document is in read-only mode, the sheet tabs won't show the insert sheet tab.
Verified in 10_10_28_Cedric.rmp build - Closing - Sophie
This is a Calc issue, therefore changed the 'Component' field appropriately.