I am having issues displaying a .shp file correctly in QGIS 2.12.3.
Sometimes when I add it to a map, it displays like the red lines in the photo below (which is how it should be). However, sometimes it displays as the gold lines in the other photo below (notice there are several lines missing in the top-center section).
I am afraid there is an issue with the geometry of the layer, but I haven't figured out how to diagnose it or fix it.
Note that the layer was created by joining together three different line layers. Originally these layers were not in the same projection, so I couldn't join them together. I thought I was able to change the layers to the same projection and join them together and thus successfully display the correct layer, but I fear I may have done something wrong.
Note that the red lines below only represents one of the original layers that was joined. So the remaining two component layers aren't always appearing as part of the combined layer as they should.