  We have investigated instability of a superconducting quantum computer by
continuously monitoring the qubit output. We found that qubits exhibit a
step-like change in the error rates. This change is repeatedly observed, and
each step persists for several minutes. By analyzing the correlation between
the increased errors and anomalous variance of the output, we demonstrate
quantum error mitigation based on post-selection. Numerical analysis on the
proposed method was also conducted.
