I ran into a problem today when loading a Form component with a couple of FormItem children. The error message said, "The Form must have form items with the same number of constraint columns". In an effort to hopefully save somebody time in the future, here is the reason this showed up.
One of my FormItem children had a skinclass defined on it, and the other didn't. The default skinclass provides three content columns (main content, sequence content and help content), while the other FormItem's custom skinclass only had two. When attempting to find the max width for each form item in a form it keeps track to make sure that all the FormItems have the same number of "content columns". If not the error will be thrown.
So to fix it I just had to make sure that every FormItem in the Form was using the same skinClass.