While setting up labeling of a line (road) layer, the labels started displaying with multiple bounding rectangles along with the text.
After trying many different labeling configurations without getting only text to display, I was able to work around by abandoning the problem project and starting over with a new project, same data, same label configuration, and they started displaying correctly. So the problem seems to be with the project itself.
Easier to show than describe. Problem project -
Correct rendering -
Looks to me like the boxes are the tracks of the placement iterations of the labeling engine. Maybe I have bumbled into some debugging setting?
Using QGIS 2.18.16 running in Ubuntu 16.04.