or! You can't use `\spacefactor' in vertical mode. \@->\spacefactor \@m
or simply! You can't use `\spacefactor' in math mode. \@->\spacefactor \@m
bite the LaTeX programmer who uses an internal command without taking “precautions”. An internal-style command such as! Improper \spacefactor. ...
\@foohas been defined or used in a private macro, and it is interpreted as
\@, followed by the ‘text’ foo. (
\@is used, for real, to set up end-of-sentence space in some circumstances; it uses
\spacefactorto do that.)
The problem is discussed in detail in “@ in macro names”, together with solutions.