I recently had an instance trading the stock X during regular trading hours where my stop (basic stop, no contingent orders or brackets in place) was not triggered. X rarely has more than 2-3 tick spread and in this case I was short and price at the best BID moved more than 5 ticks above my stop! My surprised brain quickly canceled the stop and I whacked the offer and THAT didn't work. I had to hit ABOVE the offer to finally get out. This only happened once in the 8 months I've been using IB, but it was disconcerting to say the least.