Alright. This is a discussion concerning about Sgt. Johnson in Halo: Combat Evolved.

Notice how Johnson is with you on the level Assault on the Control Room. But, we also know that Captain Keyes was on his mission to the flood containment facility, at the same time. We can note this by:

The last transmission from the Captain's dropship was from this area. That was over twelve hours ago. When you locate Captain Keyes, radio in, and I'll come pick you up.

-Echo 419 dropping off John-117 at the swamp.

As Foehammer said, the captain's last transmission was 12 hours ago, at the time John-117 was looking for Halo's control centre. So Captain Keyes's mission was on at the same time as the Chief's. And, as we can see in the cutscene of Guilty Spark, Sgt. Johnson was with the Captain. But how was that, if he was with the Chief at the same time? Was this something overlooked by bungie during the development of the game?

State your opinions.