Was clicking through the UI examples and stumbled upon this:
https://secure.phabricator.com/diffusion/P/browse/master/src/applications/uiexample/examples/PhabricatorMultiColumnUIExample.php;e8f063de25c1bdc5eccf83329d554f42008c8f44$109
Causes the `Multiple Column Layouts` example to barf out with an error.