Today, I was trying to run the executable from a gem and got the following error:

This installation of RMagick was configured with ImageMagick 6.8.9 but
ImageMagick 6.9.0-3 is in use. (RuntimeError)

My first instinct was to uninstall ImageMagick 6.9.0 and then install 6.8.9 with Homebrew. Through The Google, I learned that Homebrew saves older formula versions. You can see which versions you have like this (notice the asterisk next to the version in use):

% brew info imagemagick
imagemagick: stable 6.9.0-3 (bottled), HEAD
http://www.imagemagick.org
/usr/local/Cellar/imagemagick/6.8.8-9 (1429 files, 21M)
/usr/local/Cellar/imagemagick/6.8.9-1 (1432 files, 22M)
/usr/local/Cellar/imagemagick/6.9.0-3 (1440 files, 22M) *

Since I already had 6.8.9 installed, I could set it as my active version like so:

% brew switch imagemagick 6.8.9-9

Problem: solved.