  In the paper "The complexity of mean flow time scheduling problems with
release times", by Baptiste, Brucker, Chrobak, D\"urr, Kravchenko and Sourd,
the authors claimed to prove strong NP-hardness of the scheduling problem
$P|pmtn,r_j|\sum C_j$, namely multiprocessor preemptive scheduling where the
objective is to minimize the mean flow time. We point out a serious error in
their proof and give a new proof of strong NP-hardness for this problem.
