The cart is indeed the last box in the right column, so maybe customers didn't see the error code, but what coulde have unset the cart in the first place? I'm not sure any of the add-on modules would have any effect on that, except possibly the External Order ID Generator - but that mod fixed a major problem, so I'm really hoping that's not it.

I did a search for files with $_SESSION['cart'] in them to see if there might be any that I remember editing, and there are a couple that look a bit familiar, so will now compare them with the originals. (Although I'm not sure I'll know what to do after that!)

In the meantime, the error keeps on happening - according to the error log, 7 times today already.

[13-Nov-2007 02:04:02]
[13-Nov-2007 02:41:14]
[13-Nov-2007 03:58:58]
[13-Nov-2007 07:16:33]
[13-Nov-2007 07:18:05]
[13-Nov-2007 08:52:02]
[13-Nov-2007 09:05:06]

And that also looks a little weird - 95% of the site's users are college students, and these times seem a little too early in the morning to me. Is there any way I can record users sessions, try to match sessions with errors, then contact the users to find out what happened?

This thing really has me bugged, so I much appreciate your help!