When fine-tuning some media queries I noticed that layout changes seemed to happen at different window widths in Safari than in Firefox or Opera. When making the browser window narrower, media queries that specify a max-width kicked in a bit earlier in Safari.
The difference seemed to be around 15 pixels, so I thought it might be related to the vertical scrollbar. After reading the section about width in the Media Queries specification, I think Safari (and the other WebKit browsers I have tested in) are not following the spec, while Firefox and Opera are.