After turning on mobile web services and visiting the Moodle website with the mobile app (version 1.4), I get an email with the following error:

Subject: Insert into log failed at your moodle site Moodle
Insert into log table failed at Thursday 06th of March 2014 03:12:09 PM.
It is possible that your disk is full.

The failed query parameters are:

array (
'time' => 1394136729,
'userid' => '3',
'course' => '1',
'ip' => '13.11.4.162',
'module' => 'webservice',
'cmid' => 0,
'action' => 'moodle_user_get_course_participants_by_id',
'url' => '',
'info' => '13.11.4.162',
)

This is happening because the action field in the mdl_log table is set to varchar(40) by default and the action it is trying above is 41 characters, so it fails and emails the insert into log error.

This has happened in the past with Moodle, see:
https://moodle.org/mod/forum/discuss.php?d=164730

Note, this only happens with mobile web services enabled.