"This is a untested beta version of the exchange - use it at your own risk."

That's exactly what it is.
We are not guaranteeing any correctness or relilability yet, but we are doing our best to make this the best altcoin exchange, period (lots of work).
Markets resumed! Trading engine updated.
All trades now happen at the best possible price, even if you intentionally screw it up.
If you want to buy at 120 and at the same time there is a sell for 100, your buying price and amount will change accordingly.
If you want to sell at 100 and at the same time there is a buy for 120, your selling price will change to 120 automatically.
A critical error in the trading engine that has been resolved.
Affected users will be processed by an automatic routine that will fix some balances (less than 0.5% of users affected).
Please report any serious issues privately here or on IRC.