The only time it backfired at me is when the package you want to rollback to is not in your cache, and basically the only time it happens is for the version ins

The only time it backfired at me is when the package you want to rollback to is not in your cache, and basically the only time it happens is for the version installed during the install phase: the installer does not copy the package from the install medium to the target package cache, as it uses it straight from the medium.