The moral of the story is: don't leave your summary out of the spell-checking and/or beta-reading process. It only takes one brilliant typo (see the thrustworthy friends example) to distract the audience from what might be a perfectly adequate story. Could happen to anyone, regardless of writing skill.

The fest idea has been floated before. I don't think it would be OK to use real summaries as fic-prompts without the original authors' permission - won't do it in my journal, anyway. But some time ago (pre-HBP) we had a Summary Suckathon, in which people made up the most hellish summaries they could manage, and inflicted them on each other. It worked like the Cat's Birthday Drabblethon, and was much fun. :)

There's also a comm at LJ called perposterice - I have nothing to do with running it, but it's been known to do challenges based on SE summaries.