  In device-independent cryptography, it is known that reuse of devices across
multiple protocol instances can introduce a vulnerability against memory
attacks. This is an introductory note to highlight that even if we restrict
ourselves to device-dependent QKD and only consider a single protocol instance,
memory effects across rounds are enough to cause substantial difficulties in
applying many existing non-IID proof techniques, such as de Finetti reductions
and complementarity-based arguments (e.g. analysis of phase errors). We present
a quick discussion of these issues, including some tailored scenarios where
protocols admitting security proofs via those techniques become insecure when
memory effects are allowed, and we highlight connections to recently discussed
attacks on DIQKD protocols that have public announcements based on the
measurement outcomes. This discussion indicates the challenges that would need
to be addressed in order to apply those techniques in the presence of memory
effects (for either the device-dependent or device-independent case), even for
a single protocol instance.
