Making it more obvious that mixing edge and stable isn't supported
I think that many users aren't aware that mixing stable releases and packages from edge isn't supported and might break at any point - e.g. when we bump an soname and rebuild everything but the stable release still has the old library version in it. As such, I think it'd make sense if APK either explicitly warned you about it, or even better if it threw an error you'd have to override, so something like this wehn when doing update/upgrade/add etc. with mixed repos in /etc/apk/repositories:
$ apk update
ERROR: Detected repositories from multiple different releases in /etc/apk/repositories.
Please keep in mind that this is unsupported and might break at any point. See $SOME_INFORMATIVE_LINK
for more information. Specify --allow-broken-mixing-releases to keep going anyway.