Someone's probably going to have to think about building Sigil from source on OSX 10.9 to track down where things are going pear-shaped.
EDIT: definitely going to take a build from source... as is usually the case ... it looks like it's due to a bug in Qt5.x that has been fixed in Qt5.1.1
https://bugreports.qt-project.org/browse/QTBUG-31803