If an issue's attachment is deep-linked, and then either the attachment or its issue is deleted, JIRA logs will start to be filled with stacktraces like:

2005-10-31 00:19:57,128 ERROR [jira.web.servlet.ViewAttachmentServlet] Error finding /12152/image.gif: null
java.lang.NullPointerException
at com.atlassian.core.ofbiz.util.OFBizPropertyUtils.getPropertySet(OFBizPropertyUtils.java:23)
at com.atlassian.jira.issue.managers.DefaultAttachmentManager.getAttachment(DefaultAttachmentManager.java:63)
at com.atlassian.jira.web.servlet.ViewAttachmentServlet.getAttachment(ViewAttachmentServlet.java:46)
at com.atlassian.jira.web.servlet.ViewAttachmentServlet.getFileName(ViewAttachmentServlet.java:29)
at com.atlassian.jira.web.servlet.AbstractViewFileServlet.doGet(AbstractViewFileServlet.java:33)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:241)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:333)
at com.evermind._ha.doFilter(.:59)
....

404 responses are perfectly normal and should not result in log messages.