of a revolution of shaft 20 then the system tests whether the thread pulse counter is set to one block 180 if so then the thread pulse counter is reset to zero block 190 and the system is recycled if the thread pulse counter is at zero an output error signal is generated block 200 to indicate a skipped stitch see also fig2 c in the illustrated embodiment the retro reflective tape is a discrete element positioned on the face i e outer surface of a conventional bobbin case in an alternate form of the invention the retro reflective tape 134 may be positioned on a portion of the bobbin case that is recessed below the nominal face surface of the bobbin case so that the thread will not rub against the tape 134 and wear it off the surface in yet another form of the invention the bobbin case itself may be used as the reflective surface or target with the monitor assembly 132 positioned accordingly in yet another form of the invention the monitor assembly 132 could be a hewlett packard hbcs 1100 high resolution optical reflective sensor while described above in conjunction with the lockstitch type 301 sewing machine improper stitch.