While trying to get 5.26 released:

14:37:47 O: ======================================================================
14:37:47 O: FAIL: test_apt_source (__main__.DockerRunner)
14:37:47 O: apt source
14:37:47 O: ----------------------------------------------------------------------
14:37:47 O: Traceback (most recent call last):
14:37:47 O:   File "/builds/smcv/autopkgtest/./tests/autopkgtest", line 3085, in test_apt_source
14:37:47 O:     self.assertNotIn(' stderr ', err)
14:37:47 O: AssertionError: ' stderr ' unexpectedly found in '[ huge string redacted ]'

where the offending output is something like this:

14:21:10 E: #     autopkgtest [14:19:55]: starting date: 2022-09-20
14:21:10 E: #     autopkgtest [14:19:55]: git checkout: f2bc0dd tests: Avoid polluting stderr with sudo output

(from https://salsa.debian.org/ci-team/autopkgtest/-/merge_requests/202)

This is silly. We should have a better way to get information out of
autopkgtest than screen-scraping its stdout and stderr, which as you can
see here is prone to false-positives.

    smcv