My printed objects have horizontal holes in them (as seen I the picture below): 
This doesn't only look bad it also makes the object break at the seems.
Looking carefully at the printing process I can see that after a retraction there's a small amount of time the hotend isn't extruding plastic.
Material: PLA, Printer Robo 3D R1+, Slicer: Cura
1 Answer
(answering my own question)
The problem was the extrusion distance settings in Cura's advanced tab, reducing the value to 1.5mm solved the problem.
Other problems with the same symptoms:
- Partially blocked hotend nozzle.
- Incorrectly configured steps per mm for the extruder motor - this youtube video shows how to test and configure this.