I would use "at one point" during a narration of a past event about which I was sure of what happened and when. For example, "Yesterday Bob was rude in class. In fact, at one point he started throwing balled up papers at other students." Whereas, "at some point" I would use to narrate a past event about which I'm a little fuzzy about the details or to talk about a possible future event. For example, "Yesterday my students were out of control. At some point, paper even started flying across the classroom." (In this case, things were so crazy, I can't even pin down when the paper started flying or who started it.) Or "You might as well pay those traffic tickets now. At some point you're going to be forced to pay them anyway." I'll note for the record that if there's a hard and fast grammar rule on these, I don't know it. I'm just sharing how I use them in speech and writing. I hope it helps. :)