In an mbox mailbox, when the "From " line contains more than the
e-mail address and the date (e.g. information coming from the
"From:" header), Mutt silently ignores the message.
The issue is that there were no specifications for the mbox format
in the past (and even nowadays there are still different assumptions
about it). And I have an old mbox file where additional information
has been kept in the "From " line, and it took me some time to
discover that Mutt silently ignored the messages: I noticed the
ignored messages only after doing an external grep. This is a major
issue as one may not notice that data are missing (as this initially
happened to me), though it would occur on uncommon mailboxes (e.g.
very old ones).
I've attached an example.
Mutt should support relaxed "From " lines, e.g. just requiring that
such a line ends with a date (there could possibly be a configurable
regexp for that). And in any case, Mutt should never silently ignore
data.