Tip: Error=Error importing Web Part

Ever get this error in the logs when trying to import a webpart?:

Error=Error importing Web Part at Microsoft.SharePoint.WebPartPages.WebPartImporter.CreateWebPart(Boolean clearConnections) at Microsoft.SharePoint.WebPartPages.WebPartImporter.Import(SPWebPartManager manager, XmlReader reader, Boolean clearConnections, Uri webPartPageUri, SPWeb spWeb) at Microsoft.SharePoint.WebPartPages.WebPartPreview.get_WebPart() at Microsoft.SharePoint.WebPartPages.WebPartPreview.CreateChildControls() at System.Web.UI.Control.EnsureChildControls() at System.Web.UI.Control.PreRenderRecursiveInternal() at System.Web.UI.Control.PreRenderRecursiveInternal() at System.Web.UI.Control.PreRenderRecursiveInternal() at System.Web.UI.Control.PreRenderRecursiveInternal() at System.Web.UI.Control.PreRenderRecursiveInternal() at System.Web.UI.Control.PreRenderRecursiveInternal() at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)

Check that the class associated with your webpart is marked as public!

Feature Deploy Error

Recently I got a strange error when deploying a new feature:
Required tag 'http://schemas.microsoft.com/sharepoint/:Feature' is missing from XML file

After quite a bit of analysis I realized the following feature definition property was missing:
xmlns="http://schemas.microsoft.com/sharepoint/"