This is the draft [autosave][1] feature at work. It remembers the content of the last answer you typed until you either post the answer or start to write an answer to a different question, at which point it gets auto saved. Drafts fade away after one week.


  [1]: http://meta.stackexchange.com/questions/1177/allow-questions-to-be-saved-as-drafts-prior-to-posting