Consider following example: I use tox to run tests and see coverage in three different version of Python. For some odd reason, I've ended up with 4.2.x in python 2.7 and 3.5, and 4.3.4 in 3.6 tests and combine step.
In .coveragerc I have instruct which paths should be combined in the paths section
[paths] source = src .tox/py27/lib/python2.7/site-packages .tox/py35/lib/python3.5/site-packages .tox/py36/lib/python3.6/site-packages
When I run combine, I get output which includes all versions, and result is not combined.
I've spent hours debugging this, but accidentally spotted that I use two different version of coverage. If results of coverage are not compatible between versions, combine step should report this.
This is clearly my mistake somewhere, but nevertheless it is happened to be for second time in relatively short period of time and it would be nice if warning, or better yet, error be displayed.