I had left Evolution in offline mode and was working in another workspace when a notification popped up that I had received a new mail (judging by the sender and subject, it was from a mailing list -- I'm hard pressed to find it again, since it probably went to the corresponding mailing list folder).

I clicked on the "Go to inbox" option offered by the notification, and when I checked Evolution, it was showing a blue ribbon saying:

"Error while Unknown background operation." There was something about one of my filters not being able to run in offline mode (I don't have the precise wording anymore, since the ribbon seems to have disappeared while I was typing this).

It seemed odd that mail would arrive while in offline mode, and that it would not be possible to apply filters to it when it did.

I doubt it's rule-specific, but in any case, the failed rule looks like this:

    <rule enabled="true" grouping="any" source="incoming">
      <title>SPAM</title>
      <partset>
        <part name="header">
          <value name="header-field" type="string">
            <string>X-Spam-Status</string>
          </value>
          <value name="header-type" type="option" value="contains"/>
          <value name="word" type="string">
            <string>Yes,</string>
          </value>
        </part>
        <part name="header">
          <value name="header-field" type="string">
            <string>X-Spam-Flag</string>
          </value>
          <value name="header-type" type="option" value="is"/>
          <value name="word" type="string">
            <string>YES</string>
          </value>
        </part>
      </partset>
      <actionset>
        <part name="move-to-folder">
          <value name="folder" type="folder">
            <folder uri="email://local@local/Spam"/>
          </value>
        </part>
        <part name="stop"/>
      </actionset>
    </rule>