Bizarre edge case bug:
When replying to a tweet in which a user included their own username in the tweet the pre-populated reply will contain the username twice if when they included their own username they did not match case.
Example: https://twitter.com/#!/therizers/statuses/177885182710382594. They included their own username in their tweet, but they CamelCased it. Their actual username is not camel cased. When you press reply to that tweet it will include their username twice. Had they not CamelCased their username it would ignore it in the reply.
Essentially the JS that checks should not be case dependent and should not include the username twice in a reply.
Hope this helps. Difficult one to explain.