We have some warnings since upgrading from 2.10.0 to 2.11.0 (Final). Someone has already posted this issue on http://forum.piwik.org/read.php?2,124356,124356 but the moderator was lazy to create a report here (it's a shame).

WARNING DevicesDetection /path/to/piwik/plugins/DevicesDetection/functions.php(63): Warning - array_key_exists() [<a href='function.array-key-exists'>function.array-key-exists</a>]: The first argument should be either a string or an integer - Piwik 2.11.0 - Please report this message in the Piwik forums: [forum.piwik.org](please do a search first as it might have been reported already)