Safari will often show a cached copy of a page that is several days old, even though it showed a current version moments before. I see it most frequently on news index sites like the Techmeme river.
1. Go to site, may be in a new tab/page, view current version of page.
2. Follow a link and view linked page.
3. Hit back to return to page.
4. Page loaded is out of date rather than the version you were just looking at.
I have clearly observed this bug in Safari on iOS 5 over a dozen times now, on both an iPad 2 and iPhone 4.