During code review:

Guy (also the same guy who pushes code without making unit tests): "Hey, this thing you added is unnecessary. Remove it."
Me: "Have you read the rest of the changes? It isn't unnecessary."
Guy: "Not yet"

This happened to me once and I try and not make that same mistake again ....
At line number x a variable was created and initialised and wasn't used till after 20-30 lines and right before reaching the line where it was infact used I commented to the code writer that the variable should be removed....mere seconds later I was going "🤦‍♂️FML :/"