Created attachment 53135 [details]
Sample spreadsheet showing the problem.
If you have a sum() across multiple sheets and at least one of the terms of the sum is set empty by "" (e.g. by an other formula), then the sum() formula don't work any more. If the field is really empty (nothing in it), then sum() works.
I tried this - and it's reproducable - with 3.4.x on Linux (Fedora 16) and Windows 7.
With LO 3.3.x and OOo 3.3.0 this formula works fine.
Created attachment 53136 [details]
Screenshot of the error
*** This bug has been marked as a duplicate of bug 37860 ***