A quick one as a note to anyone who stumbles upon a counterintuitive part of aptitude that tripped me up-
You can stop aptitude from every installing a version of a package with
aptitude forbid-version package=version
And the docs mention that to clear that you use install, however they don’t mention that all install commands will clear the flag.
So for example (libavcodec52 from debian-multimedia.org breaks mpd in sid)
aptitude forbid-version libavcodec52=5:0.7.2-0.0 aptitude install libavcodec52=4:0.6.2-5 # Next aptitude upgrade will go back to broken version
Even though it seems less intuitive to me, you need to do the install (downgrade) first.