We use Eclipse as our IDE. We also use Ruby pretty extensively; we do all of our deployments and configuration using Capistrano.
The problem I've been having is with mysterious tabbing. In spite of having set my Eclipse preferences to use spaces for tabs (Preferences -> General -> Editors -> Text Editors -> Insert spaces for tabs), my files edited in the Ruby editor have been saving with tabs.
Now this normally might not be a problem, but we're not all on the same version of Eclipse, and for some strange reason, loading files with tabs in them in the Ruby editor seems to crash the IDE.
It turns out that there is yet another preference you need to set to make tabs go away. Under Preferences -> Ruby -> Editor, there is an Indentation preference. Set Tab policy to Spaces only, and you should be good to go.
Here's a handy perl one-liner to find and fix any random tabs you may have lying around in your source (I'm excluding .svn from the search, obviously you can chain together additional exclusions using multiple pipes to grep -v):
perl -i -pe 's/\t/ /gs' `find . -type f | grep -v ".svn" `