I ran into an odd error upgrading Drupal from 7.0 RC1 to 7.0. The following is the message in apache's error log:
[Mon Jan 17 13:49:47 2011] [apc-error] Cannot redeclare class queryconditioninterface in /var/www/drupal-core/drupal-7.0/includes/database/database.inc on line 1648.
The solution is to temporarily disable APC (for example, by commenting out the relevant lines in the PHP configuration. In my particular case, this in /etc/php5/conf.d/apc.ini).
PHP Version 5.3.2 Ubuntu 10.04 APC 3.1.3p1 Apache 2.2.14
Postscript. This problem seems to be caused by APC's handling of
require_once. I'd set up APC to optimize this handling, and it seems that something in either the new Drupal code, or the updated APC behaviour, is incompatible with the optimization, because the problem cropped up again with the newly installed Drupal 7 after I re-enabled APC.
For now, it appears that the fix is to turn off the include_once optimization:
apc.include_once_override = 0
The moral of the story is: turn off opcode caches and acceleration while upgrading. Is that written in the installation/upgrade instructions? Maybe it should be...