Interesting: That would suggest that if you let enough bulbs on a strand go out, you reach a cascading-failure threshold at some point, where the voltage across the remaining filaments is higher than their rating. And then I guess the breaker trips.
This means you want to replace burnt out bulbs when you notice them, rather than waiting for the whole segment to go dark.
Edit: Sorry, I misread, I thought this subthread was about parallel circuits.
If half of them burn out (and, as the article explains, become a short-circuit), then you have 50 bulbs, and each is getting 2.2V across it.