An intended mistake? 
LOL
What happyjack meant is the prime tower - it is on by default as soon as you are going for multimaterial printing with both nozzles of the dual head. The tower is used to wipe the nozzle and build up pressure again after a longer idle time, to compensate for oozing. If you switch it off, you may suffer from underextrusion, stringing and oozing blobs in the wrong places. You can play around with retraction settings to compensate for that. Since I do not like to waste material for the prime tower, the retraction is what I am trying, but others argue that a failed/dirty print is also a waste of material and prefer the tower to be on the safe side. Your decision
My settings for retraction which are good but not yet prefect: