Disabling Vsync/running the game at above 30 FPS can cause a game breaking bug. In 'Chapter 5: Lethal Devotion' Doctor Challus Mercer doesn't appear and the game can't progress. The easiest way to solve this issue was to momentarily enable Vsync in the settings. One walkthrough mentioned the bug as well. Sources will be added later.
Topic on Talk:Dead Space
I wonder if it isn't related to this