You have a git commit in your history that is causing a bug but you do not know which commit it is. Here's how to use git bisect to find the commit that causes the bug.
# in the git root, start the git bisect
git bisect start
# mark current commit as bad
git bisect bad
# tell git which is the commit is good (it can be any good commit in the far history)
git bisect good <commit-hash>
# git will tell you roughly how many revisions you need to find the bad commit
# eg. Bisecting: 24 revisions left to test after this (roughly 5 steps)
# You are now at a commit that is chosen by git bisect
# Run your test to determine if the commit is okay
# if commit is okay, then
git bisect good
# if commit is bad, then
git bisect bad
# Repeat the above until git tells you which is the bad commit
# eg. f5836a61c5e989b972499e5265760519580d3cfe is the first bad commit
# Note down your bad commit hash
# Upon finishing, then
git bisect reset
# Work on your bad commit!!!332e851383112e4505794e7c3096386868816c25