Recently, a curious bug was opened at pear.php.net for the PEAR package (#9639). In it, a user was able to uninstall the Structures_DataGrid package, even though all of its subpackages were installed, which should have prevented uninstallation.

There's been a key update to the dependency module that keeps this sort of thing from happening, but it's only in versions 1.4.5 and higher. Greg also recommends the reinstall of any packages that you've installed before and up to version 1.4.4 just to ensure that the dependency database is up to date.