  As quantum gates improve, it becomes increasingly difficult to characterize
the remaining errors. Here we describe a class of coherent non-Markovian errors
-- excitations due to an off-resonant drive -- that occur naturally in quantum
devices that use time-dependent fields to generate gate operations. We show how
these errors are mischaracterized using standard Quantum Computer Verification
and Validation (QCVV) techniques that rely on Markovianity and are therefore
often overlooked or assumed to be incoherent. We first demonstrate off-resonant
errors within a simple toy model of Z-gates created by the AC Stark effect,
then show how off-resonant errors manifest in all gates driven on a
fixed-frequency transmon architecture, a prominent example being incidental
cross-resonance interaction driven during single-qubit gates. Furthermore, the
same methodology can access the errors caused by two-level systems (TLS),
showing evidence of coherent, off-resonant interactions with subsystems that
are not intentional qubits. While we explore these results and their impact on
gate error for fixed-frequency devices, we note that off-resonant excitations
potentially limit any architectures that use frequency selectivity.
