  We investigate the implications of interference detection for experiments
that are pursuing a detection of the redshifted 21-cm signals from the Epoch of
Reionization. Interference detection causes samples to be sporadically flagged
and rejected. As a necessity to reduce the data volume, flagged samples are
typically (implicitly) interpolated during time or frequency averaging or
uv-gridding. This so-far unexplored systematic biases the 21-cm power spectrum,
and it is important to understand this bias for current 21-cm experiments as
well as the upcoming SKA Epoch of Reionization experiment. We analyse simulated
data using power spectrum analysis and Gaussian process regression. We find
that the combination of flagging and averaging causes tiny spectral
fluctuations, resulting in `flagging excess power'. This excess power does not
substantially average down over time and, without extra mitigation techniques,
can exceed the power of realistic models of the 21-cm reionization signals in
LOFAR observations. We mitigate the bias by i) implementing a novel way to
average data using a Gaussian-weighted interpolation scheme; ii) using unitary
instead of inverse-variance weighting of visibilities; and iii) using
low-resolution forward modelling of the data. After these modifications, which
have been integrated in the LOFAR EoR processing pipeline, the excess power
reduces by approximately three orders of magnitude, and is no longer preventing
a detection of the 21-cm signals.
