The mpatch_apply function in mpatch.c in Mercurial before 4.6.1 incorrectly proceeds in cases where the fragment start is past the end of the original data, aka OVE-20180430-0004.
https://www.mercurial-scm.org/wiki/WhatsNew#Mercurial_4.6.1_.282018-06-06.29
https://www.mercurial-scm.org/repo/hg/rev/faa924469635
https://lists.debian.org/debian-lts-announce/2020/07/msg00032.html