Using Git bisect to find out when a bug was introduced!

----Simplified explanation----
Here's how to use git bisect , step-by-step!
Step 1: Find a commit where things were working. ...
Step 2: Find a commit where things are not working. ...
Step 3 - N: Use git bisect to find the problem commit. ...
Step 4: Get back to a working state.
Using Git bisect to figure out when brokenness was introduced

Step by step with code.

git log -4 # find previous working commit
git checkout #hash
git bisect ?

run through previous commits then

git bisect good or bad
git checkout #hash

See link to get more info and better understanding.