Merge conflict
What happens when two branches changed the same lines. Git can't choose, so it stops and writes both versions into the file between markers, and someone has to decide what the file should really say.
Agents resolve conflicts quickly and often pick wrong, because making the markers go away is easy and looks finished. So ask what each side was trying to do. And the opposite danger is real: a merge with no conflict can still be broken, since git compares text and knows nothing about code.
Say it to your agent
“Run git config merge.conflictStyle zdiff3, then read both commits with git log -p and tell me what each side was trying to do. Keep both intents, and run the tests. If you can't tell which is right, stop and ask. Don't guess.”