I expect they're referring to either the 'low memory - close some apps' message or the 'application not responding' message. The former would make more sense. I have witnessed this quite a few times on my N950 with the beta2 firmware, often with only one or two applications running. It did not happen with the previous firmware, and I have not yet witnessed it with the latest firmware, although I've only been using the device for a few hours since the update.