  By considering an unreliable oracle in a query-based model of quantum
learning, we present a tradeoff relation between the oracle's reliability and
the reusability of quantum state of the input data. The tradeoff relation
manifests as the fundamental upper bound on the reusability. This limitation on
the reusability would increase the quantum access to the input data, i.e., the
usage of quantum random access memory (qRAM), repeating the preparation of a
superposition of `big' input data on the query failure. However, it is found
that, a learner can obtain a correct answer even from an unreliable oracle
without any additional usage of qRAM---i.e., the complexity of qRAM query does
not increase even with an unreliable oracle. This is enabled by repeatedly
cycling the quantum state of the input data to the upper bound on the
reusability.
