Thread:Extension talk:UserMerge/Database error A database query error has occurred. This may indicate a bug in the software./reply (3)

It would be useful to set those variables in LocalSettings PHP, reproduce the problem, and pasting the full error message. It should contain at least what database query failed and the cause:

$wgShowSQLErrors = true; $wgDebugDumpSql = true; $wgShowDBErrorBacktrace = true;