When I edited my answer to this question, I ended up with two answers:
Possibly an issue in the editing code?
According to the server logs, you did indeed request the inline editor, but then submitted the "Your Answer" form instead. And looking at the code, I can't see any way the submission route could accidentally become the wrong one.
I have to assume that you did really submit a second answer here by accident (maybe a back button was involved?), unless you can actually give reproduction steps for this; I'm unable to reproduce this or even come up with a remotely probable other explanation.