After editing an issue, I clicked the Create merge request button, instead of the Save changes button.
And I found something shown up in the "Merge requests" panel.
I am not a git expert and I only use its basic features to track some very simple personal projects. What should I do to fix/undo/revert it?
Do you have code changes that go along with that issue? If you were only intending to edit the issue, you should be good to close the merge request. Not sure exactly how gitlab works, but it probably created a default merge request off of master/main and there's no changes in it unless you pushed something.
To close it, open the merge request, and there should be a button at the bottom to close 
