Don't run Bundler as root.

EDIT: Lots of people seem to show up at this page wondering about this message. Here is the scoop — it usually is fine if you’re playing around with code, but bad if it’s a production environment. You’ll be doing everything with root privileges, which means anything Bundler writes is locked to root and breaks the app for every other user on the machine. To make an analogy: it’s like returning a book to the library that belongs to your boss. Do it if you have to, but you probably don’t want to make it a habit.

Bundler can ask for sudo if it actually needs it, so let it. If you keep hitting permission errors on gem install it usually means your gem path is owned by root from a previous mistake. Fix that once and move on. Also: sudo !! is still the best shortcut in the shell.