I have never run into a case sensitivity issue in Tableau 8. In order to give a better answer, it would be helpful for you to provide more info (e.g. what's your data source? what's your calculation?). A packaged workbook would be best (please read this).
Thanks for the reply Joshua. I can't load a packaged workbook, but it appears to be an issue between a live connection vs the extracted datasource (directly below). When I switch to live connection, the issue goes away (at bottom). I've replicated this a few times. Try it on your end. Someone else must have hit this.
The calc--CASE statement below--is the same except for Build Found Case-sensitive having a capital B in the string.
When 'b_6_5_5_7' then '18.104.22.168'
When 'b_6_5_5_14' then '22.214.171.124'