Journal pages that include replies to comments are broken in (some) S2 styles right now; if a page ends unexpectedly, with a message saying
Error running style: Died in S2::run_code running EntryPage::print(): Can't use an undefined value as an ARRAY reference at (eval 17601)[/home/lj/src/s2/S2.pm:203] line 2631, line 13855.
or so, you're affected. This has already been reported to LJ (many, MANY times, in fact); until they unbreak S2 styles, you can work around the problem by appending ?format=light to an entry's URL. (Be sure to do so before the fragment identifier if there is any, of course.)
EDIT, 2015-03-16: this doesn't affect all S2 themes; themes based on the Component layout are affected, others may or may not be.
EDIT, 2015-03-24: I didn't pay attention to when it happened exactly, but apparently the issue's been fixed, and everything's back to normal.