From MacFixIt;
Safari World Leaks (cont.): Caused by Saft? Yesterday we reported on an increase in the number of "World Leak" error messages generated by Safari under Mac OS X 10.3.8.
World Leaks are error messages generated by Safari's Debug menu option. They are only useful for developers, and should be of no concern to general users. In order to eliminate the messages, go back into the debug menu, and de-select the option called "Always Check for World Leaks".
MacFixIt reader Apta now reports that the presence of the shareware add-on "Saft" may be partly responsible for some of the new World Leaks.
"I have not recently been experiencing World Leaks every time I close windows in Safari 1.2.4 (v125.12). It turns out I had installed Saft 7.5.2 last week. I experimented with and without Saft and sure enough the World Leaks disappeared when I uninstalled it."