The accurev plugin sometimes misses changes that were made to a stream.
This means that the "changes" log for a project is missing entries.

With additional logging added, one finds the following entries in the log:

09-May-2011 10:05:17 hudson.plugins.accurev.AccurevSCM logCommandFailure
WARNING: History command ("C:\Program Files\AccuRev\bin\accurev.exe" hist -H accurev:5050 -fx -p Foo -s Bar -t now.1 -k move) failed with exit code 1

09-May-2011 10:05:16 hudson.plugins.accurev.AccurevSCM logCommandFailure
INFO: Not authenticated. Please use the 'login' command to authenticate.

This seems to be because, when it checks for changes, it does not log in to accurev first, meaning that it's relying on it already being logged in.
Unfortunately, accurev logins expire after a period (this was a change introduced when accurev moved from what they call "traditional" authentication to "accurev_login" authentication, which is required for the accurev web UI to work), which means that if the poll-for-changes operation is done on a slave which hasn't done any accurev building for a while, it'll try to use a timed-out login and fail.