While running acceptance tests against different databases it was detected that various PHP warning/errors and also debugging() calls are being set to apache error log. And not being sent to the browser so, obviously, behat does not catch them.
Here it's the information in the apache logs: http://pastebin.com/MKGraKt9
(I've deleted some lines about backup/restore because they log there by default in my installation).
So we should investigate those errors, warnings and debugging() calls, discover why aren't caught and of course, also, fix them.
Reproducible with both mariadb and mysql, agains current 2.7 beta+ (surely also with other databases and, perhaps other Moodle branches).