This one isn’t too hard to figure out, but annoying and frustrating. On rubygems <= 1.3 (Including Ubuntu Intrepid’s 1.3.0~RC1really1.2.0):

$ gem fetch erubis
ERROR: While executing gem ... (Errno::ENOENT)
No such file or directory - /var/lib/gems/1.8/cache/erubis-2.6.2.gem

This is a silly permissions problem. Instead, ‘sudo gem fetch erubis’.

See Rubygems bug #21134.