It could also be a, b, or c axis out of limits. Make sure they are zero referenced. There is also an error in mach3 that will do this. To get out of it, you have to make a,b, and c linear axis (uncheck angular in config). Then in homing and limits, make sure they auto zero. Then reference all home. Also make all your x,y,z softlimit max .01. You don't want your machine "0" to be on the soft limit.