Well, what should the zipfile module do when you open a broken zipfile for appending in the first place? :) There are many ways in which it could be broken. I don't think a zipfile with incorrect offsets should be treated any differently.

(I don't know *how* the zipfile module should treat broken zipfiles, and I don't know how it treats them now -- but I think the most important thing is consistency and documenting the choice :P)