Here, I use Safari 3.1.2 (5525.20.1) on Mac OS X 10.5.4 and whilst I can't reproduce the symptom today, very recently I
did see exactly the same behaviour whilst browsing
http://plone.org/support/forums and thereabouts.
Whilst preparing to submit a bug report, at that time, I either
a) refreshed the affected tabs
(AFAIR there were three or four in a row, all affected tabs originating from a command-click on a referring link)
or
b) quit then re-launched Safari.
The symptom did not recur then, and has not recurred since, so I have not submitted a bug report.
An intermittent, rare issue with WebKit, maybe?
Stefan, do you have any significant third party software that may affect behaviour of WebKit/Safari and/or windows in general? (Here I have Afloat, Saft, PithHelmet, Application Enhancer etc..)
Regards
Graham