Hadn't reviewed an inactive client site in some very long time - maybe mid-last-year.
Client has decided to make another go of it, so we dove in. Post.php is absolutely blank. Not WSOD - other pages and front-end works a-ok. Not the input field - the entire page is empty. Source shows everything is there but somehow being masked.
Running through all plugins - turned out RokTabs & RokAjaxSearch are the culprits.
Gantry Framework is up to date. Maelstrom is v1 (currently v1.4).