Looks like a Thai character with stacked diacritics. (http://en.wikipedia.org/wiki/Thai_alphabet#Diacritics)

Zalgo generator: http://www.eeemo.net/

Is there any good way to support Unicode and user-submitted content, but prevent people from drawing "outside the lines," so that for example what happened to the HN front page because of this post doesn't happen?

I guess in any case when you have user-submitted content you have to have some way to deal with inappropriate content, but something like this just makes the site's interface appear broken.

What is this, reddit?

Unicode combining diacritics. There's no real limit to the combining and it makes some text layout engines make funny characters.

Perfect example of GIGO.

I wish I could take a screenshot on my Windows Phone news reader to show you what happened :-)

S̢͎̳̞̲͈̪̳̻ͮͩt̟̳̏ͬ̔͒̈́ͦ͠a̞̤̝̟ͫ̽̂̈́ͪ͐͘n͕͐͑ͪ͐ͦ͋ͮ̅d͚̗̙̎ͫ̌â̗̬͓͍͍̳̥͆̕͠r̢̘ͣ̀d̢̢̢̘̲̺͙̂̈́̊ͬ ͎͎̫͚̣̺̤̖͊̏̀ͬ͞u̧͆ͩ́͒҉͔̠̪̖̹̠̰͎ṇ̸̛͚̟̫͎̟̣̜͋̈́ͧͯi̲̲̺͑̐ͣ͗̿̕͘͝c̦͈͇̦͈ͦ̆ͨ͝o̟̭̫̥͎̹͆́ͥ͊ͬ̏͝d̪͔̯̥̩͙̝ͩ̏͒̈́ͩ̿́̕͜ͅe͍͓̻̊͛ͅ ̸̧̻̺̤̠͙ͪ̋̽l̛̥̥ͬ͂̈́ͤ̓̀̓̚͘ͅͅͅǒ̮͓̼ͭ̂̆̇̕͘ͅl̯̯̟̗͔̳͉̰ͫ̒ͧͦͩͦ̓̓͢ͅs̝͎͚̗̮̟̒̔͛̈̊͋͒ͩͅ