When you realise a bug has been introduced somewhere in your app but you don't know how long it has been there for and/or in which part of the code it was actually written, you can use the command git bisect.
git bisect is a tool that helps you find the commit that introduced a bug. You'll need to find a commit where the code works and a commit where it doesn't.
From there, you start git bisect like this:
#start up git bisect
git bisect start
#give a commit where the code works
git bisect good a1b2c3
#and another commit where it doesn't anymore
git bisect bad d4e5f6
When you start this, it will start splitting the revisions, loading them up and asking you if the commit is good or bad.
You will need to answer either git bisect bad or git bisect good depending if the commit loaded contains the bug or not.
It will execute a binary search to help you narrow down the commit you're looking for.
You can also automate the process by executing a script against each revision that is loaded.
To do so:
#Do the same steps as above
git bisect start
git bisect good a1b2c3
git bisect bad d4e5f6
# command to run against each commit
git bisect run rspec spec/my_test.rb