Made changes in the code to debug and do not understand what's wrong. Hell, I'm on the debug production! This happened to you? To me several times. Library offers one of the ways to solve this problem.
Most everyone will understand from one picture:
This means that on the first tab we have development environment, on second stage, and third it's production(we don't need change this favicon).
It uses Tinycon - A small library for manipulating the favicon.
And one more option:
Icon will be gray on non production environment.
Link to the gem: https://github.com/accessd/rails-env-favicon