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\spacefactor complaints

The errors

! You can't use `\spacefactor' in vertical mode.


! You can't use `\spacefactor' in math mode.

or simply

! Improper \spacefactor.

bite the LaTeX programmer who uses an internal command without taking “precautions”. An internal-style command such as \@foo has 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 \spacefactor to do that.)

The problem is discussed in detail in “@ in macro names”, together with solutions.

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This is FAQ version 3.28, released on 2014-06-10.