  We prove lower bounds on the error probability of a quantum algorithm for
searching through an unordered list of N items, as a function of the number T
of queries it makes. In particular, if T=O(sqrt{N}) then the error is lower
bounded by a constant. If we want error <1/2^N then we need T=Omega(N) queries.
We apply this to show that a quantum computer cannot do much better than a
classical computer when amplifying the success probability of an RP-machine. A
classical computer can achieve error <=1/2^k using k applications of the
RP-machine, a quantum computer still needs at least ck applications for this
(when treating the machine as a black-box), where c>0 is a constant independent
of k. Furthermore, we prove a lower bound of Omega(sqrt{log N}/loglog N)
queries for quantum bounded-error search of an ordered list of N items.
