Library not loaded: /opt/X11/lib/libSM.6.dylib

When you install RMySQL, you might encounter the following error:

> install.packages("RMySQL")
...
  unable to load shared object '/Library/Frameworks/R.framework/Resources/modules//R_X11.so':
  dlopen(/Library/Frameworks/R.framework/Resources/modules//R_X11.so, 6): Library not loaded: /opt/X11/lib/libSM.6.dylib
  Referenced from: /Library/Frameworks/R.framework/Resources/modules//R_X11.so
  Reason: image not found
>

Installing XQuartz solved the problem for me.

Reference:
http://stackoverflow.com/questions/26489928/cant-load-x11-in-r-after-os-x-yosemite-upgrade