I searched to find discussion on this matter but did not find anything. I'm running Mac OS X 10.6.8 on my MacBookPro4,1. I also have Mac OS X 10.7 on another partition. Might I add that my Mac has 4 GB of memory with plenty of it free when the following happened. Yesterday I upgraded from Safari 5.0.5 to Safari 5.1. Today I began to notice strange behavior with multiple tabs, three in this case, when I spent a while on one tab (like 20 minutes) without touching the others. It looked as if Safari had removed the other two tabs from memory: when I switched to those tabs, they were initially white until they reloaded the pages they were supposed to have on them. This is the same behavior that can be seen on iOS and it's extremely irritating - it made me downgrade to Safari 5.0.5 pretty much instantly. I guess this is somehow related to the new process architecture in Safari 5.1. Have any of you experienced the same on Snow Leopard and Safari 5.1? Does it happen on Lion (I have yet to test it thoroughly)?