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The /etc/issue of my Debian testing references two different releases: stretch, which is supposed to be testing, and sid, aka unstable. What does that mean? That I have a mix of packages from the two of them?
$cat /etc/issue
Debian GNU/Linux stretch/sid \n \l
This is standard practice: in between stable releases, base-files is updated to list the next release's codename and Sid instead of a version number, and the version of base-files targeted for the release is changed to specify the version number. You can see this in the changelog, e.g.
base-files (9) unstable; urgency=medium
- Changed issue, issue.net, debian_version and os-release to read "stretch/sid", and dropped VERSION and VERSION_ID from os-release.
base-files (8) unstable; urgency=low
- Release for jessie as stable:
- Use "8" as version in /etc/issue and /etc/issue.net. As usual, this is never expected to change once that jessie is released as Debian 8.
Version 8 is the version that Debian 8 released with; it gets updated for point releases, but those changes don't appear in the main changelog (they're effectively on another branch). Version 9 is the first version for the development of Strech, the codename for the next release of Debian.
So any mix of testing and unstable, including pure testing systems with no unstable packages, will show the codename of the forthcoming Debian release, and sid, the permanent codename for unstable; thus stretch/sid in the run up to Stretch, buster/sid in the run up to Buster... The actual contents of your system are determined by your apt configuration and how often you update.
/etc/apt/sources.list?testing.