I've done some exploring and I think I've figured out that all the messages posted between 5:46 pm and 6:37 pm yesterday, May 6, were erroneously given a date of May 5. These messages were numbered from 705491 to 705546 – 56 messages in all. In some threads, this caused messages to appear out of order. Any threads that were started during that period would have seemed to disappear, because they seemed to be older than they actually were, too old to be picked up by the 1-day filter.

The problem has been fixed now, but there still are some messages with the wrong date on them. There is a thread about this in the help forum. (Click on "Help" in the upper right corner of the page.)