After creating a new site based on a custom site template that is using an alternate CSS file, you receive "An unexpected error has occurred" when trying to access Site Settings:
To fix this issue, run the following command in SharePoint 2010 Powershell:
$w = Get-SPWeb http://site/subsite; $w.AlternateHeader = $null; $w.Update()
Appears to be yet another bug in SharePoint 2010. Sigh...