Starting Emacs like this:

emacs -Q --eval "(progn (modify-all-frames-parameters '((scroll-bar-width . 7))) (custom-set-variables '(scroll-bar-mode (quote left))))"

the scroll bar is correctly display on the left, but its width is the
default, not the customized value, and it overlays the left fringe,
which is hence hidden.

If another X window is moved over the Emacs frame (at least the left
side including the scroll bar), the left fringe becomes visible (only
partly if the other X window is not moved across the whole height of the
fringe) and the scroll bar width is correspondingly narrower (I didn't
measure but it looks like the width specified above); see the attached
screen shot, which also shows the incorrect scroll bar width and hidden
left fringe in the top Emacs frame.  As soon as the lower Emacs frame
regains focus, the scroll bar returns to full width and overlays the
left fringe again.

Another display glitch conditioned by the above settings is that typing
until point or eol enters the right fringe causes the left fringe in
just the current line to become visible (and the scroll bar
correspondingly narrowed/truncated in that line); I have not succeeded
in getting a screen shot of this glitch.

These display problems do not happen when the scroll bar is either on
the right or has its default width.  They also don't happen with the
non-toolkit-scroll-bar build.  They do happen with 24.3, the earliest
version I currently have (I don't have a non-toolkit-scroll-bar build of
24.4 at hand but I guess it also has the problems).  (I haven't noticed
these problems before, since I've been using non-toolkit-scroll-bar
builds for a long time, and only since the 24.4 release have switched to
the toolkit scroll bar on the trunk to be able to test the horizontal
scroll bar.)

In GNU Emacs 25.0.50.3 (x86_64-suse-linux-gnu, GTK+ Version 3.10.9)
 of 2014-11-07 on rosalinde
Repository revision: 118312 monnier@iro.umontreal.ca-20141107162535-aiv2ipzik768ih1t
Windowing system distributor `The X.Org Foundation', version 11.0.11403901
System Description:	openSUSE 13.1 (Bottle) (x86_64)

Configured using:
 `configure 'CFLAGS=-g3 -O0''

Configured features:
XPM JPEG TIFF GIF PNG RSVG IMAGEMAGICK SOUND DBUS GCONF GSETTINGS NOTIFY
GNUTLS LIBXML2 FREETYPE M17N_FLT LIBOTF XFT ZLIB