If you’re on OS X, and you do this:

dave@home > sudo gem install mysql

And then you get this:

Building native extensions.  This could take a while…
ERROR:  Error installing mysql:
ERROR: Failed to build gem native extension.

/System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/bin/ruby extconf.rb install mysql
can’t find header files for ruby.

Gem files will remain installed in /Library/Ruby/Gems/1.8/gems/mysql-2.7 for inspection.
Results logged to /Library/Ruby/Gems/1.8/gems/mysql-2.7/gem_make.out

… it might be because you haven’t installed the XCode library yet.

And now you can rest assured, knowing that you’re not the first person to have made this mistake…