### `\``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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