When viewing a topic that has a post that was edited by user (user_id=123 in the error message below) that has since been deleted, I get the following notice twice:

[phpBB Debug] PHP Notice: in file [ROOT]/viewtopic.php on line 1585: Undefined offset: 123

The above line number is from 3.1.4-RC1 but it also happens in 3.1.3 (didn't check earlier versions).