Browse to a wiki article, click Edit, click the Link button, click Attachments, and attempt to upload an attachment. The attachment should be larger than the maximum allowed attachment size. The file will upload, then Confluence will show the user an error saying "Could not upload the file to Confluence. The server may be unavailable." No corresponding error will be written to atlassian-confluence.log at all.
The error provided to the user, and the lack of any error at all in the logs, make this difficult for an admin to troubleshoot.
The desired behavior happens if you browse to an article, go to the Attachments section, and upload the same file through the Attachments section. Confluence will display a "file too large" error to the user and will produce a "java.io.IOException: Content Length Error" in the log.