It's been a month since you last merged master code back into your feature branch... Holy crap! There are 10 conflicting files and 100+ conflicting lines of code!
Okay then; one by one you edit and commit the changes. Slowly, the conflicting number of files drops to 9, 8, 7... Finally! You've reached the final one.
It's late and you're tired. That probably was the right router method to delete right? Wait... No it wasn't! But you've already added the changes and :wq'd out of vim!
You have rerere enabled, but how are you ever going to record the correct merge info ​and​ start again on that route file? Perhaps you should just try the merge again...

Don't sweat it. You can reset just that one file to where it was at the beginning of the merge with this one weird trick!

git checkout -m <file>

So go ahead and commit all the files that were correct, then run that checkout command on the file you goofed on. Edit that sucker and commit the correct resolution, then go get a good night's sleep. You earned it.