Rails app seems to be 'requiring' a gem 'automatically'

I am running the same rails application on two different machines.I have
done a fresh 'get' from the repo into a new directory so I am sure the
code is exactly the same.

On machine it works fine.

On the other machine, it is behaving as if a certain 'require' is
missing, in particular "require 'rexml/document' " But really this
question isn't about REXML (at least I don't think so.)

So it's something about the environment. Both versions of Ruby and Rails
are identical. The gem set on each machine is different. Not sure if a
gem could be causing this but anyway, doing an exhaustive trial and
error install and remove of gems can't be the way to go.

Any suggestions on where to look? Or how to troubleshoot?

Where else is there context that could change the bahavior of a rails
app between two machines in this way?

Thanks for any tips whatsoever!!

Pito Salas