I noticed this this week as well.  I don't know how to do a backtrace
(though I'll look it up and try) but attached (and pasted below) is a
sample file.

Scot
(Edit: and the backtrace)

#+TITLE: Some Lorem

* Heading


If you do a \autocite{key_0000} followed by an org-footnote[fn:: You
seem to get an error.]  Does this happen for you too?

The error is "Invalid use of `\' in replacement text."

If you do a \autocite{key_0000} and break the line
like this [fn:: No trouble now.]  everything's fine.  I notice this
because I use long-lines mode.

And I just realized that I do this kind of thing with some
frequency:[fn:: See the fine discussion of this and other relevant
matters in \fullcite{bibtexkey_0000}]  Do I have any hope that that
might be made to work?  At the moment, it throws the same error.  If I
need to I can use LaTeX footnote for this kind of thing, since I
already do  live with the limitations that imposes (no HTML export).


----------------------
Backtrace:

Debugger entered--Lisp error: (error "Invalid use of `\\' in replacement text")
  replace-match("\\[1]")
  org-footnote-normalize(nil t)
  org-export-preprocess-string(#("#+TITLE: Some Lorem\n\n* Heading
\n\n\nIf you do a \\autocite{key_0000} followed by an
org-footnote[fn:: You seem to get an error.]  Blah here.\n\nThe error
is \"Invalid use of `\\' in replacement text.\"\n\nIf you do a
\\autocite{key_0000} and break the line \nlike this [fn:: No trouble
now.]  everything's fine.  I notice this because I use long-lines
mode.\n\nAnd I just realized that I do this kind of thing with some
frequency:[fn:: See the fine discussion of this and other relevant
matters in \\fullcite{bibtexkey_0000}]  Do I have any hope that that
might be made to work?  At the moment, it throws the same error.  If I
need to I can use LaTeX footnote for this kind of thing, since I
already do  live with the limitations that imposes (no HTML
export).\n\n\n\n\n\n" 0 19 (fontified nil font-lock-fontified t
:org-license-to-kill t) 19 20 (fontified nil :org-license-to-kill t)
20 21 (fontified nil) 21 23 (fontified nil) 23 31 (fontified nil) 31
34 (fontified nil) 34 138 (fontified nil) 138 347 (fontified nil) 347
746 (fontified nil) 746 751 (fontified nil)) :emph-multiline t
:for-LaTeX t :comments nil :tags not-in-toc :priority nil :footnotes t
:drawers nil :timestamps t :todo-keywords t :add-text nil
:skip-before-1st-heading nil :select-tags ("export") :exclude-tags
("noexport") :LaTeX-fragments nil)
  org-export-as-latex(nil nil nil "*Org LaTeX Export*")
  org-export-as-latex-to-buffer(nil)
  call-interactively(org-export-as-latex-to-buffer)
  org-export(nil)
  call-interactively(org-export nil nil)