Question: Once in a while, some well-meaning user appends the string " [closed]" to a question title after receiving a good answer. This is, of course, confusing to people who, for example, visit the question later and don't see a "closed as *reason* by *voters* at *time*" box.

I can't think of any case where this would be a desired behavior, so I propose rejecting any title that ends in " [closed]" except for the ones generated by the system for actually-closed questions. A quick explanation under "Oops! Your question couldn't be submitted because:" should be enough to make this feature non-confusing.

Answer: 
You're right, there's no reason to ever allow this — added a check for it.

EDIT:  

The filter has been updated to check for the strings "[migrated]", "[on hold]", and "[duplicate]" as well.
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> How often does this actually happen?
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There are 29 questions on SO that currently (as of Oct 31 data dump) have something like this in the title (I figured [closed], (closed), and {closed} would cover the vast majority of them), and an additional 39 questions where that same pattern has been edited out at one time or another. Only one of those 68 questions is actually closed.

<http://odata.stackexchange.com/stackoverflow/s/653/closed-questions>

I think editing the questions and leaving a comment (to say to leave it alone, except if it's a duplicate, in which case flag for mod attention) is probably sufficient.

**EDIT:** I went in and manually edited the offending posts. I think I got all of them. Unfortunately we'll have to wait until January to see that reflected in the December data dump.