The 1.20 RC will often cause periodic hangs which last several seconds and completely bring the computer to a halt. The mouse and keyboard do not respond during this time and the entire computer is effectively frozen until each hiccup passes. Now, if this was infrequent or didn't cause the computer as a whole, including all other programs and the OS itself, to stutter on a regular basis, it wouldn't classify as critical, but this is occurring so frequently on my setup even when running nothing else in the background to contribute to it, that it is not only disrupting the use of SL but it is actually forcing me to pause while I type this report out because the keyboard has stopped responding twice because of such hiccups while I was doing so. So it completely disrupts the use of my computer as a whole, every few minutes, often four or five times over the course of 2 or 3 minutes, which makes it qualify for critical status since SL is effectively gimping the whole of the computer while it is running. If it is relevant, this seems to usually not show up until the viewer has been open for a while, though with the sometimes long periods between the usually-clustered instances of the effects, this could be coincidental.