There should be exactly one form in production for {ObjectTypeCode}

I had this error crop up today - the key phrase seemed to be:
"There should be exactly one form in production for 10009" and, per the URL in the error message it seemed to be triggered on a request for an icon for a custom entity. Strange.

Event code: 3005
Event message: An unhandled exception has occurred.
Event time: 3/26/2010 11:41:30 AM
Event time (UTC): 3/26/2010 3:41:30 PM
Event ID: 7a05487c52a4410ca2d7c0388566b66f
Event sequence: 124
Event occurrence: 1
Event detail code: 0

Application information:
Application domain: /LM/W3SVC/2/ROOT-4-129140911906159536
Trust level: Full
Application Virtual Path: /
Application Path: C:\Program Files\Microsoft Dynamics CRM\CRMWeb\
Machine name: CRMSERVER

Process information:
Process ID: 3764
Process name: w3wp.exe
Account name: NT AUTHORITY\NETWORK SERVICE

Exception information:
Exception type: InvalidOperationException
Exception message: There should be exactly one form in production for 10009

Request information:
Request URL: http://CRMSERVER:5555/ORG/_Common/icon.aspx?objectTypeCode=10009&iconType=NavigationIcon&inProduction=1&cache=1
Request path: /ORG/_Common/icon.aspx
User host address: ::1
User: DOMAIN\User
Is authenticated: True
Authentication Type: Negotiate
Thread account name: NT AUTHORITY\NETWORK SERVICE

Thread information:
Thread ID: 3
Thread account name: NT AUTHORITY\NETWORK SERVICE
Is impersonating: False
Stack trace: at Microsoft.Crm.Application.Platform.Customization.RetrieveInProductionOrganizationUIByOrgId(Guid organizationId, Int32 objectTypeCode)
at Microsoft.Crm.Application.Pages.Common.Icon.GetIconBits(Int32 objectTypeCode, IconType iconType, Boolean inProduction)
at Microsoft.Crm.Application.Pages.Common.Icon.ConfigurePage()
at Microsoft.Crm.Application.Controls.AppUIPage.OnPreRender(EventArgs e)
at System.Web.UI.Control.PreRenderRecursiveInternal()
at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)

A little digging showed that we had an incorrectly registered step in a plugin. Looking at the plugin registration tool showed that there was a step registered for one of our plugins for a Primary Entity of 'none'. I think that while I was registering a step for pluging, I hit the tab key and the step window disappeared. I just opened a new Step registration dialog and proceeded on my happy way. Next time I'll be more careful!