This is an important distinction between (funcall #'foo ...) and (funcall 'foo ...) in Common Lisp as well. #'foo evaluates to the function named by the symbol foo, while 'foo evaluates to the symbol foo itself. So if you redefine the function named by the symbol foo any already compiled forms like (funcall #'foo) will still use the old version but (funcall 'foo) will use the new version.

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The above explanation is wrong; whenever #'foo is evaluated it will resolve to the current function slot for the symbol foo. Since the form (funcall #'foo) evaluates #'foo each time, it will naturally be affected by changes to the function-slot for foo. If you bind a variable to #'foo (like e.g. a route in a routing library as TFA does for clojure) then the evaluation happens when you bind the variable. See my reply to throwaway1x31 for a full example showing the difference.