What I think happened here is that, given we both have 2k+ rep privileges and our edits apply automatically, your edit "fell over" mine, as some sort of cumulative edit.
Maybe you where in the middle of an edit when mine got processed, as we can see they have aprox. 2 minutes difference between our edits:
We could even blame Caching here, and probably saw a "previous" version of that answer when you clicked on edit. Anyways, both edits served good to the question for readability and remark on the main course of action.
Edit: Another way of thinking of it could be like a Git Version Control, which could explain the "merge" between our edits, something like:
- I submit my edit, adding some line breaks (say line 3) and italics (say, on line 6). This is equivalent to making git commit and push
- While you were editing, my change takes place. Something like git pull from your side.
- You submit your edit, adding bolds to that same "line 6" I added italics. As that edit is more thorough in that part (**'s is more than *'s) and also because you have 2k+ edit privilege, it takes place immediately (with no conflicts). This is equivalent to your git commit and push
- This resulted in a second edit where the line breaks (line 3) were edited and also the bolds (line 6) with its more recent version (yours).
Browsing a bit, I found something to back that "git" hypothesis on an answer given by Jeff on the post Preventing conflicting simultaneous edits, extracting from it:
It is specific to the area in question, so one user can edit body and you can edit tags without conflicting. (Or title, etc). It's only when you both edit the same field at the same time this error is returned on save.