Arch GNU+Linux, Kernel 4.18.5
Broken: 2.79.6, Git 19c65cd (branch master)
Worked: None that I'm aware of
Short description of error
If you have a scene with multiple rigged objects, the armatures and meshes are incorrectly parented upon exporting then importing them. Rather than there being two armatures at the root of the scene with each one containing a single mesh, there will be (if you have two objects) one armature at the root, the other armature as a child of it, with both meshes attached to the inner-most armature.
Note that this certainly seems to be a fault of the importer and not the exporter: I've manually reviewed the exported model and it appears fine. Additionally, upon attempting to import a Collada file generated by custom software I've got the exact same problem as importing the file generated by Blender.
Exact steps for others to reproduce the error
Start with an empty scene, and create a cube. Add a single bone. Make the cube a child of the bone, using the armature deform with automatic weights option (not using any weights prevents this problem from occurring).
Repeat this a second time.
Export the scene as a Collada file.
Open a new instance of Blender, and import the scene with Collada.
The objects will have incorrect parents, and this has consistently occurred each time I have tried it:
Here are the screenshots of the scene, before exporting (matches attached blend file):
And after export/import:
Sample Blender file before export: