Look at this report:

https://taskotron.fedoraproject.org/artifacts/all/c754fb76-22f6-11e8-843d-525400fc9f92/tests.yml/elfutils-0.170-4.fc26.log

In that ABI change report, task-abicheck says two functions as being removed, but the author of the package actually added those functions. So, presumably, task-abicheck is calling:
"abipkgdiff new-package old-package", rather than doing:
"abipkgdiff old-package new-package".

Looking at the source code of task-abicheck, at https://pagure.io/task-abicheck/blob/develop/f/download_rpms.py, we see at line 29, we have this:
'target_dir': stable_rpmsdir
and at line 41, we have this:
'target_dir': update_rpmsdir

I think those two line are backward, it should be the other way around; that is, "update_rpmsdir" at line 29 and stable_rpmsdir at line 41.

This seems to be so since commit https://pagure.io/task-abicheck/c/b0c00bf25f001a327c8f24c3ebf92824ee0e04ef?branch=master, from Kamil.

@kparal: What do you think?