I'm not sure I've fully understood your explanation,I do reckon that attempting to mix different sources or adding PPAs can/will eventually lead to troubles, my original question however was about the theoretical possibility of rolling back just to the previous version of a given package,using apt-get with the command
which as you point out should be used with care,but nonetheless should work in principle.
In reality,it doesn't:I've tried some random packages with the --simulate option and some can apparently be downgraded to some specific version numbers but not to other versions (E: Version 'xxx' for 'package' was not found),some can't be downgraded at all.
Maybe it has do do with dependencies,maybe for security reasons packages such as web browsers can't be downgraded.
Pinning of course will work,but you have to do this before upgrading the system-unfortunately you won't know if a program won't work as you expect until you've actually upgraded it,and at that stage you can't actually undo the last update,which is what I was theoretically after.