I seem to have found a bug with the Block Editing feature in New Blog Templates. I'm not sure how you guys flag a post or page as uneditable, but it seems to break some filters.
Another plugin I use (User Switching) hooks to the 'user_has_cap' filter which is supposed to pass an array of capabilities to the hook function, but with editing blocked by the New Blog Templates it somehow ends up passing a boolean (false) sometimes. This filter is supposed to always pass an array, and since that plugin strongly types the parameters, it throws a fatal error.
On my site, I noticed it when my theme tries to use the edit_post_link() template tag while the current logged in user doesn't normally have editing capability on the post.