My take on the "Cannot add the specified assembly to the global assembly cache (...)" issue

I got this when updating a WSP, on a SharePoint 2013 server. Tried the suggested IISReset, Timer Service restart... no luck. This however worked like a charm:

[System.EnterpriseServices.Internal.Publish] $publish = new-object System.EnterpriseServices.Internal.Publish
$publish.GacRemove("c:\windows\microsoft.net\assembly\remaining-path-to-trouble.dll")

Publishing content types, 'site could not be retrieved'

"Job: MetadataSubscriberTimerJob skipped processing a site 'http://meSite' which could not be retrieved."

I got this while trying to push published content types, into a site collection that I had previously moved to a different content database, but without performing the necessary IISReset.

Even after the IISReset, I still got the same error ... but a restart of the Timer service goes a long way.