When using Views Calc with an exposed filter, if the query yields no results, I get this warning.

Warning: array_flip() [function.array-flip]: Can only flip STRING and INTEGER values! in DrupalDefaultEntityController->load() (line 178 of /Applications/MAMP/htdocs/appname/includes/entity.inc).

This does not happen in the views ui, only on the page view. It does not seem to cause any critical problem, it is just there.