ActiveRecordStore using wrong session_id value?

Background: Using ActiveRecordStore to manage session data. The
session_id for a request will occasionally not match the session_id
passed to the database backend, causing all subsequent requests to
fail. This is sporadic and cannot be reliably reproduced.

Example from logs:

Processing CommunityController#signin (for 9.65.2x.x at 2006-10-18
05:06:30) [GET]
  Session ID: 2a3a53c1c1d2db8a94ffb7eb708df35f
  Parameters: {"action"=>"signin", "controller"=>"community"}
Rendering within layouts/community
Rendering community/signin
Completed in 0.01856 (53 reqs/sec) | Rendering: 0.01831 (98%) | DB:
0.00000 (0%) | 200 OK [http://localhost/community/signin]
TypeError: wrong argument type false (expected Data): SELECT * FROM
sessions WHERE (session_id = '3bf9ddbf9110b9bac12cf960333c90e3') FETCH
FIRST 1 ROWS ONLY

The session_id 2a3a53c1c1d2db8a94ffb7eb708df35f is correct and valid.
The value in the SQL is not - doesnt exist in the database at all.

Any clues?