Under some circumstances (not yet fully understood) the index of a mbox can be corrupted and the messages shown in the message view are out of sync with those shown in the message list.

For example, a folder contains some messages (msg1, msg2, msg3 & msg4), some of which are moved to an other folder (e.g. by an antispam filter or a message rule).

The remaining mails shown in the message list in the folder are msg1, msg3 & msg4. This is the intended behavior since only msg2 was removed in this example.

Clicking on msg1 displays msg1 in the message view, but clicking on msg3 will show msg2 and clicking on msg4 will show msg3. If the messages are copied or moved to another folder the message list will show msg1, msg2 & msg3. Hence msg4 is lost unless some more data corruption occurs.

Typically the corruption will show up only the next time evolution in run after the messages have been moved, and it seems that deleting ".ev-summary" and ".ev-summary-meta" files can temporarily resolves the issue, but do not prevents some further data corruption to occur.

There is no known method to reproduce this bug, but it has been reported at least on Ubuntu and Fedora, see:
https://bugs.launchpad.net/ubuntu/+source/evolution/+bug/220014
https://bugzilla.redhat.com/show_bug.cgi?id=442926