Custom list. Custom New/Edit/View forms. Everything works perfectly... until the latest round of updates.
Now if you Bingle this error message, you'll get about twenty possible causes that run the gamut from a known bug, an unknown bug, and fixed/not fixed/hotfixed/workaround bug, all the way to corrupt content types and invalid form data.
In my case, however, it turned out to be a misspelled column name in a FormField control.
I'm just glad it's Friday.