  We present a detailed comparison of the fixed-order predictions computed by
four publicly available computer codes for Drell-Yan processes at the LHC and
Tevatron colliders. We point out that while there is agreement among the
predictions at the next-to-leading order accuracy, the predictions at the
next-to-next-to-leading order (NNLO) differ, whose extent depend on the
observable. The sizes of the differences in general are at least similar,
sometimes larger than the sizes of the NNLO corrections themselves. We
demonstrate that the neglected power corrections by the codes that use global
slicing methods for the regularization of double real emissions can be the
source of the differences. Depending on the fiducial cuts, those power
corrections become linear, hence enhanced as compared to quadratic ones that
are considered standard.
