You can easily forget if someone else (not yourself) did this (they merge the branch and reverted it), later when you try to merge the branch it could be successful, without any merge conflicts, but still won't lead to the expected changes.
OK I suppose that is more likely, but I still think this is easily-avoidable through process. If you're closely-enough involved with a ticket that you were going to be the one merging it, I think it's reasonable to expect that you have an idea of things that have happened in the story.
