From the "No Good Deed Goes Unpunished" file - After the fix-a-fellow-cyclist's-flats (yes, two of them) episode, I rode to work, worked a full day, started home and noticed my rear tire soft. Not flat, just way softer than it should have been. Dang. A Vittoria sew-up too, so I couldn't just patch it. Fortunately I carry two spares and they are easy to replace. I guess it was losing air through a tiny puncture, but it could have been through the valve stem. I haven't had time to think about it. Latex tubes leak air anyway, but not that fast.