Normally I clean my spam folder every 2-3 days. The same was this morning. In the evening I noticed that I already had ~40 spam messages. One hour later number of spam messages increased to >250, which seemed to me a bit odd. So I looked through the folder and found there messages dating back to October! What was the most dissapointing thing is that on November 1 I received a job invitation, but gmail filtered it as spam and showed only today :(weird...

I have two main Gmail accounts. One for personal use and another that I use to sign up for mailing lists of activist organizations. I get around 30-40 emails per day and it's hard to catch up. I don't clean out my spam because sometimes Gmail mislabels good mail as junk but not too often. I just go into my spam folder and find any messages that aren't spam and report it to gmail to get it into my inbox, then I select all of the unread spam messages and mark them as read. It's been a while now since the alst time Gmail has mismarked anything as junk. (I've never deleted a single email since the opening of my Gmail account).

Anyways, I haven't checked my activist email account in around two weeks now and today I logged in and I had 501 new emails in my inbox, and almost 800 unread messages in my spam folder. When I went into my spam folder, I clicked "Older >" to get to where the last new messages were and I noticed that it stopped at around 500 when it said there were ~800 unread messages. So I kept on clicking on "Older >" and sure enough, some old junk mail had respawned as unread.

Very, very strange, indeed. This might explain the sudden jump in spam. http://blogoscoped.com/forum/77144.html

Someone else reported they suddenly had spam mails in their inbox.. but those mails were from October!

And a German blog post on this problem:
http://blog.patrickkempf.de/2006/hat-google-mail-schluckauf/

hey.. all of a sudden my spam count jumped like crazy.. went from like 600 something to 918.. and a bunch of them are old messages... crazy thing