Since upgrading to 2.7 I noticed that the Tippy plugin causes a blank screen after a guest user leaves a comment.
I have temporarily resolved the issue by commenting out the following two lines in tippy.php:
/* add_filter('comment_text', 'tippy_getspot', 4);
add_filter('comment_text', 'tippy_display', 40); */
Fingers crossed an update/fix for this issue is just around the corner.