  This is an erratum to the article: "Computation of maximal projection
constants" (J. Funct. Anal., 277). The statement of Lemma 3.1(2) of that paper
is incorrect. As a consequence of this the proof of Theorem 1.4 is incomplete.
In this erratum we prove a corrected version of Lemma 3.1 and explain why, with
this weaker result, our original strategy for proving Theorem 1.4 no longer
works. The other results of the article, in particular the alternative proof of
Gr\"unbaum's conjecture, do not depend on Lemma 3.1 and are thus not affected
by this error.
