  Current-day quantum computing platforms are subject to readout errors, in
which faulty measurement outcomes are reported by the device. On circuits with
mid-circuit measurements and feedforward, readout noise can cause incorrect
conditional quantum operations to be applied on a per-shot basis. Standard
readout error mitigation methods for terminal measurements which act in
post-processing do not suffice in this context. Here we present a general
method for readout error mitigation for expectation values on circuits
containing an arbitrary number of layers of mid-circuit measurements and
feedforward, at zero circuit depth and two-qubit gate count cost. The protocol
uses a form of gate twirling for symmetrization of the error channels and
probabilistic bit-flips in feedforward data to average over an ensemble of
quantum trajectories. The mitigated estimator is unbiased and has a sampling
overhead of ${\sim} 1 / (1 - 2 r)^m$ for $m$ total measurements and
characteristic readout error rate $r$ per measurement. We demonstrate the
effectiveness of our method, obtaining up to a ${\sim} 60\%$ reduction in error
on superconducting quantum processors for several examples of feedforward
circuits of practical interest, including dynamic qubit resets, shallow-depth
GHZ state preparation, and multi-stage quantum state teleportation.
