I've experienced this at times on Area 51. Usually, you make a post, then go to edit the post. On submit, the following error:

[phpBB Debug] PHP Warning: in file [ROOT]/phpbb/notification/manager.php on line 359: Invalid argument supplied for foreach()

General Error
SQL ERROR [ mysqli ]
Unknown column 'Array' in 'where clause' [1054]

Appears to have something to do with the notification methods introduced recently.