Don’t overwrite Rails’ built-in instance variables

So I'm hammering away at a project tonight, writing a few specifications for a module.  I've changed very little - or so I think - when five of my specifications start reporting this error:

NoMethodError in 'LoansController POST 'create' with valid parameters should succeed'
undefined method `env' for

This happens on the line where I call post :create in a controller spec.   Undefined method 'env'?   What's that about? I'm certainly not trying to call a method named "env".

It took me a little bit to figure out what was going on.  See, this series of tests needed access to a particular LoanRequest object I was pulling out of fixtures.   So I'd put above the tests:

before(:each) do
... some other stuff ...
@request = loan_requests(:johns_loan_request) # fetch fixture
end

Well, kids, it just so turns out that it's a bad idea to overwrite the @request instance variable in any rails context. Who knew?

Come to think of it, it would be nice to change the accessibility and/or mutability of Rails' basic instance variables and classes to prevent this kind of accidental overwrite by the programmer. Because when you make that mistake, it's invariably a bit of a pain to figure out because the error it causes is obscure.

Maybe I'll have to dig into the code one of these days to see if anything can be done about it.