When processing committed changesets, for each modified/added/removed/renamed file, webrev attempts to add a comment line indicating the changeset in which the files was involved.

This happens in the comments_from_mercurial() function.

This function eventually calls some form of hg log --follow - such as:
hg log --rev <rev1> --rev <rev2> --rev .... -- follow --template 'rev {rev} : {desc}\n' <file>

It appears that this command does not work when the file in question doesn't exists (because one of the changesets removed it).

The consequence of seeing the 'abort: cannot follow file not in parent revision: <file>' when calling webrev is however benign: the final webrev is still workable, but the link to the removed files don't show any revision information (so you don't know which changeset removed them).

Presumably comments_from_mercurial() might be improved by first checking whether the file ($logf) exists, and if it doesn't, it could call hg log -v for each revision to figure out which revision should be listed for this file...