  Results on the thermal transition of QCD with 3 degenerate flavors, in the
lower-left corner of the Columbia plot, are puzzling. The transition is
expected to be first-order for massless quarks, and to remain so for a range of
quark masses until it turns second-order at a critical quark mass. But this
critical quark mass and resulting "pion" mass disagree violently between Wilson
and staggered fermions at finite lattice spacing, and decrease sharply with the
lattice spacing, for staggered fermions at least. To clarify this puzzle and
eliminate potential systematic effects from rooting, we study the 4-flavor
theory with staggered fermions, on lattices with 4 to 10 time-slices. Our
results are qualitatively similar to the 3-flavor case, so that rooting is not
an issue. However, dramatic cutoff effects are visible, even on our finest
lattices. Universality implies that cutoff effects for Wilson fermions are even
more dramatic. In order to obtain a first-order thermal transition in the
continuum theory, extremely light quarks are needed.
