I've experimented with spiral vase mode in s3d and I get strange misbehavior. The printer seems to ignore z axis movement in certain conditions, starting to screech one z layer over another without actually raising the head from layer to layer. After poking around I found out these conditions: it seems to happen on curvy detailed models which render to lots of short gcode line segments. A cube or low poly facetted vases print perfectly, while wooman busts or a simple cylinder get wrecked (and cancelled manually before damaging the nozzle). I have inspected the gcode and it seems perfectly good for any model, z value slowly increasing between each segment. I think there is some edge-case error in printer firmware, specifically gcode processing. I can't find any other reason for a cylinder vase fail while a cube vase prints correctly.