Soviet pilots shot down Americans in Korea, and Soviet anti-air operators shot down Americans in Vietnam. I'm sure it happened at other times too, and I'm sure the reverse happened at times as well.
American pilots being shot down in Ukraine might lead to WW3, but in any case it wouldn't be because article 5 was broken in the first instance. We can talk about escalations, obviously, and how they would lead to a strike on Nato soil, but that's not quite the same. If it were, then we could make the same argument about supplying arms leading to article 5, or sanctions, or all the other things Russia are whining about.