Visit https://meta.wikimedia.org/w/index.php?title=Schema:Sandbox&action=edit and add a property

"key": "value"

to the outer object.

It should fail with a clear error stating that additional properties are not allowed. Instead, it shows:

[4fefd1e3] 2013-01-29 17:47:56: Fatal exception of type MWException

Version: unspecified
Severity: normal