Someone in the Apple forums reported that when using Safari to view certain web sites, all accented Latin characters were being replaced by Hebrew. Normally the culprit for such behavior should be a non-Unicode Hebrew font with the same name as that required by the web page code, which was Arial in this case. But it seemed that no such font could be found anywhere on the machine.
It turns out that the old Windows font Web Hebrew AD, with the filename wehad.ttf, is in fact just such an animal, replacing accented Latin by Hebrew according to the Win-1255 encoding. When examined with a font editor, you can see that its internal name is actually Arial, so that Safari can't tell the difference. Mystery solved.