When you use Yank, it is important to keep in mind what Yank was designed to do. Yank is designed for removing new software. It may not be able to reverse changes made by an upgrade installer. Here's the difference:
Yanking new software · When you install new software, the installer places many new files on your hard drive. Yank is very good at removing these new files and you can be confident that removing the new files will restore your system to its earlier state.
Downgrading existing software · When you install a new version of an already-installed program, the installer usually modifies a lot of existing files on your hard drive. It may also add some new files to your hard drive, but most changes will be modifications, not additions. Yank is not able to reverse a file modification, because it doesn't have a copy of the older version. Therefore, yanking a software upgrade will probably not give you a functioning copy of the old version of the software. More likely, it will give you a crippled hybrid version of the software.
Therefore, there are certain packages and installers that Yank will not be able to remove reliably. OS X system upgrade packages, for example, will rarely or never yank reliably because they modify files and Yank is unable to reverse the modifications.
In summary, use Yank when installing a new program that you want to be able to remove completely at a later date. Do not depend on Yank to be able to downgrade your computer to an older version of a particular software package.