From time to time our Linux 2.6 kernel generates the following message:

  TCP: Peer 1.2.3.4:64259/51542 unexpectedly shrunk window \
          4197231805:4197240525 (repaired)

It was a bit scarier back then when it read something like this:

  TCP: Treason uncloaked! Peer 1.2.3.4:64259/51542 shrinks window \
          3166327388:3166327393. Repaired.

The message has been modified in December 2008. While there are quite a lot of posts and articles on this topic, a pretty good explanation can be found in commit 2ad4106: Clear stale pred_flags when snd_wnd changes.