PivotBySubtable error since 2.7.0 update

Since my update to 2.7.0 I get this error message:

There is an error. Please report the message (Piwik 2.7.0) and full backtrace in the Piwik forums (please do a Search first as it might have been reported already!).

Notice: Undefined index: UndoPivotBySubtable in /users/amote/www/analytics/core/Translate.php on line 173

please check that all files were uploaded properly in particular files in lang/ directories

also try delete tmp/* folders and try again?

Ah, deleting of contents of tmp helped a lot. Thank you (tu)
But I wonder, why data in tmp could make such a hassle.