Create a wiki page. Select Creole format and make sure you are in WYSIWYG edition mode.
Write "Second level < First level" and format this as a first level heading (using the format combobox). Copy the text and paste under the heading.
The pasted text is the HTML version instead of the creole one: <meta http-equiv="content-type" content="text/html; charset=utf-8"><h2>Second level < First level</h2>