forex markets - no limit up/down
futures markets - has limit up/down
stock markets - has limit up/down
In theory, BTC is most like currency trading. When the market gets more established, limit up/down will not be needed.
it is not a limit! when there is suspicios activity, it shutsdown. they check it if there is any, hacks/cracks/cheating performed. if not they resum the market. if cheating is perfomed, they fix the problem, and do rollback.