When rendering animation to an existing path with already rendered frames in it those are all deleted when you are rendering to the same filenames. This means you cannot easily rerender just a few frames of your animation, say fr.5-10 of a 250fr sequence. All 250 is deleted and replaced by 5-10... Desired behaviour would be to delete only those frames which are rerendered.
*** This bug has been marked as a duplicate of bug 377354 ***