Steps to Reproduce:
Navigate to https://wikimedia.org.au/wiki/Special:UserMerge
Enter two usernames (I did Steven Zhang as old and Steven Crossin as new), click delete all user and then click merge user
Merge fails, error returned says
A database query error has occurred. This may indicate a bug in the software.[f4498650747ce11f31bdc55e] 2019-06-13 07:34:54: Fatal exception of type "Wikimedia\Rdbms\DBQueryError"
UserMerge expected to be successful.