This was a stupid mistake on my part. Late on May 12 EDT, I was trying to figure what was causing the delay in notifications for clubs. When someone makes a comment on the wall of a club you belong to (or responds to a comment you made), you should receive a notification for that within two minutes at most. Instead, the notification was not showing up for up to 20 minutes.
Since the notification database doesn't get purged until I purge it and since I hadn't done this in a while, I figured that might be the cause. There were well over 2,000,000 entries at that point, so it seemed reasonable that the size of that database was the issue. So, I ran a code to delete all notifications that were older than a week.
On May 13, some users began noticing that notifications older than 24 hours were no longer available. I looked into this on May 14, and found that when running the aforementioned code, I used the wrong timestamp. Instead of using the timestamp for a week ago, I used the one for what was then the current time.
My deepest apologies for this error. It was a stupid mistake, and something that should not have happened. I can also promise that it will not happen again as I've now set up a script that will automatically delete old notifications; keeping that database lean and removing the possibility of me using the wrong timestamp again.
