Great tip if you ave issues removing WSP solution packages and web parts from third parties and find they don't install or uninstall properly.  You may also have trouble activating or deactivating features in site settings wen these invalid features crop up.

Errors that could be shown in SharePoint when installing or activating a feature "An unexpected error has occurred", "Failed to compare two elements in the array" or various other error messages may be shown while activating, deactivating, or browsing to a recently failed deployment of a WSP solution .

To check or remove invalid features on SharePoint 2007 site:

1. First, run the WssAnalyzeFeatures tool and view the Problems Log for invalid feature id (guids)
2. Try running STSADM -o deactivatefeature -id %featureIdFromLogFile% -force
3. Run the WssAnalyzeFeatures tool and see if the problems we're resolved.
4. If feature can't be removed or is in a subsite for a failed WSP solution uninstall...try running the WssRemoveFeatureFromSite to forcefully invalid old features.
5. Remove the feature ID's until WssAnalyzeFeatures shows no errors.

Here is a snippet of Scott's Blog for more information:

The usual reason for this problem is that some sites in the site collection have features assigned where the feature is not installed in the server farm. The main problem here is that you cannot easily identify which features are missing as the missing features are not reported in the error message.

To overcome this problem I have written a tool which allows to identify all features used in a site collection which are missing on the server: WssAnalyzeFeatures.

To resolve the problem you then have to install the identified missing features on the exporting server. In case that this is not possible you have to remove the features from the site collection or the affected sites. Usually this can be done using STSADM -o deactivatefeature but sometimes fails if the feature definition is not installed on the server. In this case you can use WssRemoveFeatureFromSite.

Stefan Goßner : MOSS - Common Issue - Content Deployment fails with "Failed to compare two elements in the array."