\spacefactor complaints

The errors 

! You can't use `\spacefactor' in vertical mode.
\@->\spacefactor 
                 \@m
or 
! You can't use `\spacefactor' in math mode.
\@->\spacefactor 
                 \@m
or simply 
! Improper \spacefactor.
...
bites the LaTeX programmer who uses an internal command without taking "precautions". The problem is discussed in detail in "@ in macro names", together with solutions.

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