Odin arriving when you're trying to Draw in FFVIII, Critical hits when a monster has 5 hp remaining, Supernatural Border on touhou 7 just before a boss.
What are the little helpers in games that just get there when you least want them?
edit: also when Donald casts stop in the Colleseum on KHI when you're trying to do time trial and you have to wait for it to wear off for the damage to take effect. And then once you reach the rock titan you beat him and while the whole slow motion thing at the end of boss battles is running your time runs out, Knowing if Donald didnt cast that stop you mightve made it!