After going through the webpart and doing a little QA on the webpart itself, I found out what was going on.
The site in question was set in a Turkish language. Apparently there is a different way of displaying an 'i' or 'I', and is converted to a Turkish 'ı'. When the webpart goes to combine the search request with selected document types, it was converting all of the document type class names into the Turkish 'ı' if there was an 'I' in the naming of said class.
We were able to move around this dilemma by extracting the string that dealt with the document type classes, reconverting their class names back to English - US, and the web part now works as expected.
Again, this issue was found in version 6.0, so I'm unsure if it exists in v7 or v8. The file where this was happening at was located in ~\CMSModules\SmartSearch\Controls\SearchResults.ascx.cs