I had an issue with the quickfix functionality in Vim and the Rails Plugin.

In test failures, a path is given to the test file where there was the test failure. This path starts sometimes in my Installation (Rails 2.3.2, Ruby 1.8.7) with a '/' (only happens paths to test cases, I think). I don't now why, it doesn't really make sense and the vim-rails plugin doesn't like this, too and try's to open up a file with an absolute path instead of one relative to the rails project path.

Today I was so annoyed by this (after each 'Rake test' call a new tab opens up with an empty file) that I went for fixing it. The fix is a little change to /usr/share/vim/autoload/rails.vim.

Somewhere in there, there are the following lines: (line 941+ here)
" stack backtrace is in brackets. if multiple lines, it starts on a new line.
let s:efm=s:efm
\.'%Ctest_%.%#(%.%#):%#,'
\.'%C%.%#\ [/\\?%f:%l]:,'
\.'%C\ \ \ \ [/\\?%f:%l:%.%#,'
\.'%C\ \ \ \ %f:%l:%.%#,'
\.'%C\ \ \ \ \ %f:%l:%.%#]:,'
\.'%C\ \ \ \ \ %f:%l:%.%#,'

You need to change Lines 4 and 5 of that section to:
\.'%C%.%#\ [/\\?%f:%l]:,'
\.'%C\ \ \ \ [/\\?%f:%l:%.%#,'

This will test for an optional leading slash and ignore it so that vim opens up a relative path from now on. Perhaps other changes are needed, I'll update this article if I encounter sth.

UPDATE
I noticed there are more problems with the rails plugin here. Most of the time, the error message isn't found. So I spent a few hours figuring out these odd vim error formats. It's now working for the failures I had till now.
How to apply my changes:

Look for the lines containing '" Current directory' and 'function! s:makewithruby(arg,bang,...)' and replace the area in between with the following code:

" Current directory
let s:efm='%D(in\ /\\?%f),'
" Failure and Error headers, start a multiline message
let s:efm=s:efm
\.'%A\ %\\+%\\d%\\+)\ Failure:,'
\.'%A\ %\\+%\\d%\\+)\ Error:,'
\.'%+A'."'".'%.%#'."'".'\ FAILED,'
" Syntax errors in the test itself
let s:efm=s:efm
\.'%.%#.rb:%\\d%\\+:in\ `load%.%#'."'".':\ %f:%l:\ syntax\ error\,\ %m,'
\.'%.%#.rb:%\\d%\\+:in\ `load%.%#'."'".':\ %f:%l:\ %m,'
" And required files
let s:efm=s:efm
\.'%.%#:in\ `%.%#require'."'".':\ %f:%l:\ syntax\ error\,\ %m,'
\.'%.%#:in\ `%.%#require'."'".':\ %f:%l:\ %m,'
" Syntax Errors
let s:efm=s:efm
\.'%A%.%#.rb:\\d%\\+:in\ `%.%#'."'".':\ %m\ (%.%#Error),'
" Exclusions
let s:efm=s:efm
\.'%C%.%#(eval)%.%#,'
\.'%C-e:%.%#,'
\.'%C%.%#/lib/gems/%\\d.%\\d/gems/%.%#,'
\.'%C%.%#/lib/ruby/%\\d.%\\d/%.%#,'
\.'%C%.%#/vendor/rails/%.%#,'
" Specific to template errors
let s:efm=s:efm
\.'%Ctest_%.%#(%.%#Test):%\\?,'
\.'%C%.%#Errors\\?:\ %#%m,'
\.'%C\ %\\+On\ line\ #%l\ of\ %f,'
\.'%C%\\(You\ might\ have\ expected\ an\ instance\ of%.%#%\\)%\\@=%m,'
\.'%C%\\(The\ error\ occurred\ while\ evaluating\ %.%#%\\)%\\@=%m,'
\.'%C%\\(%.%#didn'."'".'t\ change\ by%\\)%\\@=%m,'
\.'%C%\\(%.%#expected\ but\ was%\\)%\\@=%m,'
\.'%Z%\\(<%\\d%#>.\\)%\\@=%m,'
" stack backtrace is in brackets. if multiple lines, it starts on a new line.
let s:efm=s:efm
\.'%C\ %\\{5}/\\?%f:%l:in `test_%.%#'."'".']:,'
\.'%Z\ %\\{4}/\\?%f:%l:in `%.%#'."'".','
\.'%Ctest_%.%#(%.%#)\ [/\\?%f:%l]:,'
\.'%C\ %#[/\\?%f:%l:in `test_%.%#'."'".','
" Catch all
let s:efm=s:efm
\.'%Z\ %\\{4}/\\?%f:%l:%.%#,'
\.'%Z%.%#from\ %f:%l,'
\.'%Z%.%#from\ %f:%l:%.%#,'
let s:efm=s:efm
\.'%Z%m,'
" Another catch for syntax errors in tests
let s:efm=s:efm
\.'%A%f:%l:\ %m\ for\ %.%#(%.%#Error),'
\.'%Z\ %#from %.%#.rb:%\\d%\\+:in\ `load%.%#'."'".','
" Exclusions
let s:efm=s:efm
\.'%-G%.%#/lib/gems/%\\d.%\\d/gems/%.%#,'
\.'%-G%.%#/lib/ruby/%\\d.%\\d/%.%#,'
\.'%-G%.%#/vendor/rails/%.%#,'
\.'%-G%.%#%\\d%\\d:%\\d%\\d:%\\d%\\d%.%#,'
" Final catch all for one line errors
let s:efm=s:efm
\.'%f:%l:\ syntax error\,\ %m,'
\.'%-G%\\s%#from\ %.%#,'
" Drop everything else
let s:efm=s:efm
\.'%-G%.%#'

let s:efm_backtrace='%D(in\ /\\?%f),'
\.'%\\s%#from\ /\\?%f:%l:%m,'
\.'%\\s#{RAILS_ROOT}/%f:%l:\ %#%m,'
\.'%\\s%#[/\\?%f:%l:\ %#%m,'
\.'%\\s%#/\\?%f:%l:\ %#%m'

function! s:makewithruby(arg,bang,...)

Wohooo! This is paradise for a regex lover like me =)