Failed to update App Service Plan the sites exceed maximum number of slots

After some testing I wanted to scale down my 5 instances of Standard S3 plan machines and I got this error when scaling down to FREE:

 Failed to update App Service plan : {"Code":"Conflict","Message":"The site(s) '' exceed maximum number of slots allowed for the hosting plan. Remove all deployment slots before scaling to a different mode.","Target":null,"Details":[{"Message":"The site(s) '' exceed maximum number of slots allowed for the hosting plan. Remove all deployment slots before scaling to a different mode."},{"Code":"Conflict"},{"ErrorEntity":{"Code":"Conflict","Message":"The site(s) '' exceed maximum number of slots allowed for the hosting plan. Remove all deployment slots before scaling to a different mode.","ExtendedCode":"04054","MessageTemplate":"The site(s) '{0}' exceed maximum number of slots allowed for the hosting plan. Remove all deployment slots before scaling to a different mode.","Parameters":[""],"InnerErrors":null}}],"Innererror":null}

Basically the answer is in the exception.  But if you only read the first part then you miss that is explains the issue as well.

Simply, I had some deployment slots and deployment slots are not offered in the plan I was scaling to.  So, I had to either delete the slots or scale to a plan where they are supported.  You can see the features of each App Service Plane here.  You can also read my description of an ASP here.