Multiple Johnsons
Did you realize that Sergeant Johnson can appear four times in the level? That's certainly the case on Legendary anyway; I haven't checked the other difficulty levels but I wouldn't be surprised if they're the same.
For starters he's in the mess hall as you enter it after the bridge cutscene. He's down the right, well out of the action until covie reinforcements appear through the two far doors. But in the meanwhile another Johnson may have appeared behind the door near the initial two Elites and a Grunt. You'll get either Stacker or Johnson there. So, you can actually end up with two Johnsons in the mess hall battle. You may need to work fast to save the one on the right though, as he's liable to come under fire quick once the reinforcements flood in.
A little further on you come to the first airlock, where there's a Johnson standing at an airlock hatch. Not a great place to stand; he gets blown up every time. Finally, when you eventually come to the stairs, you'll find either a Stacker or Johnson.
All in all, that means you could've encountered four Johnsons in the level! The guy sure does get around a bit eh? It seems like some pretty crazy scripting from Bungie - especially having him blown up at the airlock. You'd think they would've just used nondescript Marines for that. As well as dying at the airlock, he can die in the other places. So, one level into the game, Johnson could've been killed four times already (see my talk-through for some pictures). Tough cookie!