Recently we found out that all the layouts were completely broken in Safari 5.1.7, the browser found from older Macs and also from some Windows (like mine). I already experiences some problems with the webfonts (CORS issues) and first expected that this would also relate to the fonts.
But it did not. Our minified and combined CSS files seemed to hit the (unknown) limitation of maximum CSS line length. Unfortunately YUI Compressor provides an option to provide maximum line length with --line-break handle. By running YUI Compressor with --line-break 5000 the CSS rules were loaded properly also in Safari 5.
I could not find any references what is the exact maximum line length. However, experimenting that 5000 works but 6000 did not anymore work would give some hint.
Update: I asked about the max line length in Stack Exchange's Pro Webmasters Q&A service.