I tried to do an stsadm -o export today on our Production farm and got this lovely error:

[7/28/2008 11:53:56 AM]: FatalError: Failed to compare two elements in the array.
   at System.Collections.Generic.ArraySortHelper`1.QuickSort[TValue](T[] keys, TValue[] values, Int32 left, Int32 right, IComparer`1 comparer)
   at System.Collections.Generic.ArraySortHelper`1.QuickSort[TValue](T[] keys, TValue[] values, Int32 left, Int32 right, IComparer`1 comparer)
   at System.Collections.Generic.ArraySortHelper`1.Sort[TValue](T[] keys, TValue[] values, Int32 index, Int32 length, IComparer`1 comparer)
   at System.Collections.Generic.ArraySortHelper`1.Sort(T[] items, Int32 index, Int32 length, IComparer`1 comparer)
   at System.Array.Sort[T](T[] array, Int32 index, Int32 length, IComparer`1 comparer)
   at System.Collections.Generic.List`1.Sort(Int32 index, Int32 count, IComparer`1 comparer)
   at System.Collections.Generic.List`1.Sort(IComparer`1 comparer)
   at Microsoft.SharePoint.Deployment.WebSerializer.GetDataFromObjectModel(Object obj, SerializationInfo info, StreamingContext context)
   at Microsoft.SharePoint.Deployment.DeploymentSerializationSurrogate.GetObjectData(Object obj, SerializationInfo info, StreamingContext context)
   at Microsoft.SharePoint.Deployment.XmlFormatter.SerializeObject(Object obj, ISerializationSurrogate surrogate, String elementName, Boolean bNeedEnvelope)
   at Microsoft.SharePoint.Deployment.XmlFormatter.Serialize(Stream serializationStream, Object topLevelObject)
   at Microsoft.SharePoint.Deployment.ObjectSerializer.Serialize(DeploymentObject deployObject, Stream serializationStream)
   at Microsoft.SharePoint.Deployment.SPExport.SerializeObjects()
   at Microsoft.SharePoint.Deployment.SPExport.Run()
*** Inner exception:
Object reference not set to an instance of an object.
   at Microsoft.SharePoint.SPFeature.EnsureProperties()
   at Microsoft.SharePoint.SPFeature.get_TimeActivated()
   at Microsoft.SharePoint.Deployment.WebSerializer.ExportFeatureComparer.System.Collections.Generic.IComparer<Microsoft.SharePoint.Deployment.ExportObject>.Compare(ExportObject exportObject1, ExportObject exportObject2)
   at System.Collections.Generic.ArraySortHelper`1.QuickSort[TValue](T[] keys, TValue[] values, Int32 left, Int32 right, IComparer`1 comparer)

A bit of digging found this on the Microsoft Support page. Not much help if you don't know what Feature is not installed but still in use on a Site Collection! Stefan has an excellent console application you can install on the farm to report directly on this which can be found on MSDN Code site.  The reports generated will give you the Feature ID Guid of the Feature that is being referenced that is no longer installed.

To deactivate the rogue Feature you can use:

STSADM -o deactivatefeature -id 945a186c-846e-63ff-bb89-e94098c21943 -url http://localhost -force

Another alternative is Steven Van De Craen's Windows Form version which more or less does the same thing but I found that it couldn't see the Site collection when I put in the URL.