I have no idea how I caused ListForm not to exist but after some clumsy tinkering with a custom schema.xml file, the main section of NewForm.aspx page was coming up blank and the AllItems.aspx page for all lists in all sites across my dev farm refused to render the normal menu.
The SharePoint log file was telling me:
Control template "ListForm" does not exist
Examining a list’s NewForm.aspx file in SharePoint Designer (which of course makes me feel dirty all over) you’ll note it holds a ListFormWebpart. “Control template” would further point me to the _controltemplates virtual directory—mapped to \12\TEMPLATE\CONTROLTEMPLATES, but this directory doesn’t contain a ListForm.ascx or ListFormWebpart.ascx file.
Thankfully the Application event log entry was more precise:
Load control template file /_controltemplates/DefaultTemplates.ascx failed: The file '/_controltemplates/DefaultTemplates.ascx' does not exist.
A quick check of \12\TEMPLATE\CONTROLTEMPLATES and of course DefaultTemplates.ascx was nowhere to be found. Copy/paste from a functional server and we’re back in business. Now to figure how what caused it to be deleted in the first place…