A few months ago I released a rough first version of an ASP.NET control to display text-links served by LinkLift.
Today I found quite a subtle bug that never showed up before (since I never really had the control on my site for more than a few hour): I used the FileSystemInfo.CreationTime to keep track of the last time the local copy of the XML was saved. But the CreationTime is the first time the file was saved, because all the subsequent saves are only treated as updates, and the CreationTime is never changed again. This caused the control to download the XML definition every time it was loaded. So I changed it to FileSystemInfo.LastWriteTime, which is, as the name itself tells, the last time the file has been saved.