Mystery stacktraces: IllegalArgumentException in CustomFieldTypeModuleDescriptor.getEditHtml

I have seen this in a few people's logs now:

2006-07-31 15:10:35,250 INFO [atlassian.jira.upgrade.ConsistencyCheckImpl] Starting JIRA Enterprise Edition, Version: 3.6.2-#156
2006-07-31 15:10:35,250 INFO [atlassian.jira.upgrade.ConsistencyCheckImpl] Configured to use database: hsql
2006-07-31 15:10:35,250 WARN [atlassian.jira.upgrade.ConsistencyCheckImpl]

******************************************************************************************
hsqldb is an in-memory database, and susceptible to corruption when abnormally terminated.
DO NOT USE IN PRODUCTION, please switch to a regular database.
******************************************************************************************

2006-07-31 15:10:35,328 INFO [atlassian.jira.upgrade.ConsistencyCheckImpl]
2006-07-31 15:10:35,343 INFO [atlassian.jira.upgrade.ConsistencyCheckImpl]

*****************************************************************************************************
JIRA 3.6.2 build: 156 (Enterprise Edition) started. You can now access JIRA through your web browser.
*****************************************************************************************************

2006-07-31 15:10:36,203 WARN [webwork.dispatcher.ServletDispatcher] Unable to find 'webwork.multipart.saveDir' property setting. Defaulting to javax.servlet.context.tempdir
2006-07-31 15:10:53,109 INFO [jira.action.admin.DataExport] Export took 15109ms
2006-07-31 15:10:53,109 INFO [jira.action.admin.DataExport] Wrote 247318 entities to export
2006-07-31 16:57:09,562 ERROR [webwork.util.ServletValueStack] METHOD: "fieldHtml", exception:
java.lang.IllegalArgumentException
at com.atlassian.jira.plugin.customfield.CustomFieldTypeModuleDescriptor.getEditHtml(CustomFieldTypeModuleDescriptor.java:93)
at com.atlassian.jira.issue.fields.CustomFieldImpl.getEditHtml(CustomFieldImpl.java:645)
at com.atlassian.jira.issue.fields.CustomFieldImpl.getBulkEditHtml(CustomFieldImpl.java:675)
at com.atlassian.jira.web.action.issue.bulkedit.BulkWorkflowTransition.getFieldHtml(BulkWorkflowTransition.java:655)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:585)
at webwork.util.InjectionUtils$DefaultInjectionImpl.invoke(InjectionUtils.java:61)
at webwork.util.InjectionUtils.invoke(InjectionUtils.java:52)
at webwork.util.ValueStack.findValue(ValueStack.java:426)
at webwork.util.ValueStack.findValue(ValueStack.java:175)
at webwork.view.taglib.WebWorkBodyTagSupport.findValue(WebWorkBodyTagSupport.java:61)
at webwork.view.taglib.BasicPropertyTag.doStartTag(BasicPropertyTag.java:54)
at org.apache.jsp.secure.views.bulkedit.bulkworkflowtransition_005feditfields_jsp._jspx_meth_webwork_property_31(org.apache.jsp.secure.views.bulkedit.bulkworkflowtransition_005feditfields_jsp:2305)
at org.apache.jsp.secure.views.bulkedit.bulkworkflowtransition_005feditfields_jsp._jspx_meth_webwork_if_8(org.apache.jsp.secure.views.bulkedit.bulkworkflowtransition_005feditfields_jsp:2005)
at org.apache.jsp.secure.views.bulkedit.bulkworkflowtransition_005feditfields_jsp._jspx_meth_webwork_iterator_3(org.apache.jsp.secure.views.bulkedit.bulkworkflowtransition_005feditfields_jsp:1945)
at org.apache.jsp.secure.views.bulkedit.bulkworkflowtransition_005feditfields_jsp._jspx_meth_webwork_iterator_2(org.apache.jsp.secure.views.bulkedit.bulkworkflowtransition_005feditfields_jsp:1806)
at org.apache.jsp.secure.views.bulkedit.bulkworkflowtransition_005feditfields_jsp._jspx_meth_webwork_else_1(org.apache.jsp.secure.views.bulkedit.bulkworkflowtransition_005feditfields_jsp:1257)
at org.apache.jsp.secure.views.bulkedit.bulkworkflowtransition_005feditfields_jsp._jspService(org.apache.jsp.secure.views.bulkedit.bulkworkflowtransition_005feditfields_jsp:434)
at org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:97)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:802)
.....