Saxat från What's wrong with extending the DOM.
In some versions of Safari 3.x, there's a bug where navigating to a previous page via back button wipes off all host object extensions. Unfortunately, the bug is undetectable, so to work around the issue, Prototype has to do something horrible. It sniffs browser for that version of WebKit, and explicitly disables bfcache by attaching "unload" event listener to window. Disabled bfcache means that browser has to re-fetch page when navigating via back/forward buttons, instead of restoring page from the cached state.
Härligt, eller vad? Sajter som går tungt bör snarast byta ut Prototype om de råkar köra det.