Dual extrusion problem with Cura-15.04.4 and CuraEngine-15.04.

Dual extrusion problem with Cura-15.04.4 and CuraEngine-15.04.

I am taking my first steps with dual extrusion and Cura and have noticed a problem for which I cannot determine a workaround.

A skirt is printed that encompasses the print and wipe tower at upper-left. I am finding that occasionally, during the print, the wipe tower material is deposited outside the skirt at a distance from the wipe tower location equal to the nozzle separation.

This does not happen on every wipe tower usage but it is obviously an error.

Has anyone seen this happening? Perhaps @Daid_Braam can offer an insight as to what is going on.