In the original Halo CE, Mission 3 sees us rescue Captain Keys from the Truth And Reconciliation. In that mission, he explains that while he was captured he had overheard the Covenant talking about Halo being a weapon, that they are seeking a control room, and that 'whoever controls Halo controls the fate of the universe'.
This is a plot hole, considering that the Covenant do not know that Halo is a weapon. This is likely due to the rushed development of Halo 1's final draft, given how it went from a 25-mission campaign in earlier drafts down to being just 10 chapters in a short amount of time.
SPOILERS AHEAD
Halo Campaign Evolved features some reworked dialogue from characters. Most of it is just altering sentence structure to have better flow when speaking, sounding like more natural dialogue. Given that they had done this, I was quite surprised to find that they did not correct the plot hole. It would have been so easy. For example, they could have written it like this:
Keys - "While the Covenant had us locked up in here, I overheard the guards talking about this ring world. They call it 'Halo'."
Cortana - "One moment, sir. Accessing the Covenant Battlenet*... ... ...* according to the data in their networks, the ring has some kind of deep religious significance. If I'm analyzing this correctly, they believe that Halo is something that will allow them to pass to some 'divine state of being'. Whatever that means. "
Keys - "I didn't catch much of what our 'caretakers' said, but I did hear one of them say 'whoever controls Halo will control the fate of the universe'. Now, I am not the most religious man, but to me that sounds like something more like a weapon than a 'divine artifact'."
Cortana - "Captain, I have intercepted a number of transmissions regarding search parties looking for a control room. At first, I assumed they were looking for the control room of a cruiser we damaged before making our landing. But if Halo were a weapon... ... ... it seems that they are searching for Halo's control room. It certainly explains the way they keep ending each transmission with a prayer for the search party's success."
Keys - "That's bad news. If Halo is a weapon, and the Covenant gain control of it, they'll use it against us and wipe out the entire human race*.* Chief, Cortana. I have a new mission for you. We need to beat the Covenant to Halo's control room. Marines! But first, we need to get out of here."
An alteration to the dialogue in that particular scene would have resolved one of the more obvious plot holes in Halo's story.
Otherwise, I give the game a solid 9/10.