Today I wanted to move a site collection into another content database, because a request came from the customer to raise the site collection quota to 200GB.
As you know the suggested content database size is 200GB and therefore we decided to allocate a new content database for it.
Move-SPSite cmdlet timed out and the site remained in the original content database. The second shot went wrong because of the site collection already exists in new database, however the site stay in place.
Checking of the content database, it showed the site count not incremented, but on the SQL server the content database AllSites table showed me the site collection id.
The easiest way to remove this site collection id from here you have to run the next one-liner script.
Get-SPContentDatabase <databasename> | %{$_.ForceDeleteSite("<siteid>",$false,$false)}