I got a bug report yesterday in which my package was broken because lexical
scoping was suddenly in effect.

After some init-file-bisecting, this is the conclusion:

> It appears as though your first instinct about lexical-binding is
correct. I had a ;;; -*- lexical-binding: t -*- directive in my
init-packages.el file, which is responsible for downloading the installing
missing packages (of which multiple-cursors.el is one).
>
> It seems that if the file which calls package-install has that flag
turned on, it applies the lexical binding to the compiler, which results in
the following compiler warning:
>
> In mc/region-strings:
> mc-mark-more.el:78:20:Warning: assignment to free variable `strings'
>
> This warning doesn't appear in the compiler messages if lexical-binding
is nil.

This is on Emacs 24.2.1 on OS X. Full compiler warnings and code excerpts
can be found here:

https://github.com/magnars/multiple-cursors.el/issues/39

- Magnar Sveen