Jul 232015
Very important:

Please make sure necessary backups are in place before you perform this operation. First test in your dev/test environment and only then use in your production environment.

Ideally a feature will be scoped to a Farm, WebApplication, Site, or Web, but orphaned features won’t be having a scope. The command to list all orphaned features in a SharePoint farm is as follows…

PowerShell command to list all orphaned features…
Get-SPFeature | ? { $_.Scope -eq $null }
Sample output…
PS C:\windows\system32> Get-SPFeature | ? { $_.Scope -eq $null }

DisplayName                    Id                                       CompatibilityLevel   Scope                         
-----------                    --                                       ------------------   -----                         
ReportServerCentralAdmin       5f2e3537-91b5-4341-86ff-90c6a2f99aae     14                                                 
ReportServerStapling           6bcbccc3-ff47-47d3-9468-572bf2ab9657     14                                                 
PowerView                      bf8b58f5-ebae-4a70-9848-622beaaf2043     14                                                 
ReportServerCentralAdmin       5f2e3537-91b5-4341-86ff-90c6a2f99aae     15                                                 
ReportServerStapling           6bcbccc3-ff47-47d3-9468-572bf2ab9657     15                                                 
PowerView                      bf8b58f5-ebae-4a70-9848-622beaaf2043     15
PowerShell command to delete an orphaned feature…
foreach($f in @(Get-SPFeature | ? { $_.DisplayName -eq "ReportServerCentralAdmin" }))

Why did we need a foreach? Because a feature could be listed twice with different “CompatibilityLevel” values. So in this case “ReportServerCentralAdmin” is listed twice with “CompatibilityLevel” set to 14 and 15. So we delete both via $f.Delete().