This isn't the case with Oanda.
While they do not guarantee exact stop levels, they do guarantee that you cannot lose more than what your account balance was, therefore no negative equilibrium.
See here:
http://fxtrade.oanda.com/legal/risk-warning
Following Oanda Forum I've seen myself instances in which in such situations at least the equity equilibrium of the trader was restored by Oanda.
So it should be part of a good risk management to have at least as much reserve money in a different account as exactly what your trading account balance...