How to catch erroneous SQL operator
In this instance, if you wanted to use XE you can grab it through "sqlserver.error_reported". The error number for a subquery returning more than one value is 512 - so we can filter on ONLY error numbers of 512.

XE Session Code:

CREATE EVENT SESSION [Error512Tracking] ON SERVER 
ADD EVENT sqlserver.error_reported(
    ACTION(sqlserver.client_hostname,sqlserver.database_id,sqlserver.sql_text,sqlserver.username)
    WHERE ([error_number]=(512))) 
ADD TARGET package0.event_file(SET filename=N'C:\ExtendedEvents\Error512Tracking.xel',metadatafile=N'C:\ExtendedEvents\Error512Tracking.xem')