Yahoo! Calendar (and Yahoo! in general) has some really unrefined (read as: bad!) user experience. Here is the latest I ran across. I tried to accept a Y!Calendar invitation, but instead was given this page, which reads:
Yahoo! Calendar Problem (code 41) Your Calendar is temporarily inaccessible. Please try again in a few minutes.
Of course, my calendar was fine (I could view/edit/add events manually). There are so many better ways for Yahoo! (or anyone else) to handle this error, but in a world where online software is never ‘complete’ and always ‘being patched’ or ‘updated’, the user experience is rarely ideal.