Steps to reproduce:
- Do a clean install of D7, using the standard profile
- enable the ctools, page_manager, and panels modules
- in page manager, enable the node view, and create a panels variant
- Under content, add the following node components: title, body, comments, comment form
- Save the variant
- add a node, with comments enabled
- type in the comment body field, and press 'preview'
- WSOD! (after execution timeout)
Needless to say, preview works fine when the page manager variant is not present/disabled.