I admit that this post is purely idle curiosity. But I recently stumbled upon a little factoid saying that the TeX error "Interwoven alignment preambles are not allowed" has the following explanation in the manual
If you have been so devious as to get this message, you will understand it, and you will deserve no sympathy.
This tickles my funny bone in the right way, but it also scratches my curiosity cat behind the ears. Does anyone know what real reason for this error message, and why it's treated as so utterly abhorrent?