Episode 309: Who saved Usopp on the bridge at Enies Lobby?
In the manga Usopp was saved by Zoro, but in the anime it seems Usopp was saved by a stray Rankyaku Gaichou shot from Lucci. He got extremely lucky that it hit the three of them.

Below are screenshots from chapter 309. At 15:43 Lucci releases the shot. At 15:46, Luffy dodges the shot and it goes upwards. At 15:56 it hits the Marines next to Usopp.