Browser developers might not be fully aware of potential privacy leakage when instrumenting browser engine for those functionalities and hence, either causing users browsing activities in private mode be persisted to disk (violation of afore­mentioned stealthiness goal) or causing previous browsing data be carried over to private session (violation of aforementioned fresh­ness goal).
it is not uncommon to see the pattern shown in Table 3 in the browser s release cycle: a new web standard is mis­implemented by developers, causing a privacy leakage bug which takes years to be patched.