  Quantum walk search may exhibit phenomena beyond the intuition from a
conventional random walk theory. One of such examples is exceptional
configuration phenomenon -- it appears that it may be much harder to find any
of two or more marked vertices, that if only one of them is marked. In this
paper, we analyze the probability of finding any of marked vertices in such
scenarios and prove upper bounds for various sets of marked vertices. We apply
the upper bounds to large collection of graphs and show that the quantum search
may be slow even when taking real-world networks.
