proposed question title:

• What are causes of the error messages "There's no line here to end"?

there are myriad questions with this error message in the title, and nearly as many conditions that result in the error. a single question with all possible conditions covered would be a valuable addition.

the present question is posted as community wiki, with an invitation to list the conditions that trigger the error.

known conditions triggering this error:

• ending a paragraph with \\.

• use of \\ at the beginning of a component of \maketitle (and possibly elsewhere) (reported in Problem with \maketitle: "there's no line here to end" )

• use of \\ in beamer, triggered at \end{frame} (reported in There's no line here to end. \end{frame} )

• use of \newline{} at the end of the last line of an element in moderncv (reported in \cventry ! LaTeX Error: There's no line here to end )

• including text in a graphics file (reported in https://tex.stackexchange.com/a/259021 )

please add other situations that aren't already covered by this list.