When I say I've read the books a gazillion times, I'm only sort of exaggerating. Okay, I'm exaggerating a lot, but I really do reread the series at least once a year, usually two or three times, depending on how busy that year is. This is the first time I've made this connection. Maybe I'm a little slow, but whenever I see a post or listicle with Potter facts, I'm all over them, and I can't recall ever coming across anyone saying anything about the Snape - Expelliarmus connection.
When you think about it, it's kind of a big deal. It's probable Harry would have learned this spell eventually, but he learned it from Snape, a man he despised until the very end. It's a tiny, but powerful connection. Even when it appeared as though Snape was doing everything in his power to screw up with Harry, he was still saving him, and it was because of that spell.
Also, when you consider Harry used "Expelliarmus" to kill Voldemort, one could say Snape not only helped betray old Voldy, but he had an indirect impact on his death. Yes, he was playing the double agent for Dumbledore, but I think of that role as a third-degree-to-saving-Potter, where this is more of a second-degree-to-saving-Potter.
Maybe I'm reading more into this than I should. After all, the thought just apparated into my head, but I like it. What do you think?