I've done this on three different machines (both PPC and Intel), and the bug is always reproducible:
1) Open a window in Safari with multiple tabs
2) Hide the address bar/toolbar
3) Load (or reload) a page in a new tab
4) Select a different, existing tab and reload the page (using the key command or menu command)
The result: the page in the previously selected tab loads into the currently selected tab. The only way to prevent this is to reload pages with the toolbar visible. On my Powerbook, I always (or usually, due to this bug) use my browsers with the address bar/toolbar hidden, as it saves a good deal of space. I've sent bug reports to Apple since I first noticed this bug (when Safari 3 Beta was first released), but I haven't heard or seen any reports from anyone else... I'm certainly not the only one experiencing this, am I?