Under certain circumstances, it is useful for a user to be able to create a horizontal frame with a negative width. (An example would be when an incipit showing the original clefs and time signatures of late-Renaissance vocal music is needed before the modern version of the score begins.)
However there are a number of issues in this process. Please see the following thread for details: http://musescore.org/en/node/93591
The issue is to enable horizontal frames to be selected by the usual 'click-on...' procedure even when they are numerically narrower than 0.0 sp. The work-around proposed in the above-mentioned thread is functional, but it is more complex than most users would be able to figure out without help.