What if Snape does not have to maintain any cover?

[annemehr]

No: HPFGUIDX 101989

Pippin: 
> Once 
> the lessons begin there's not one reference  to Harry's father, till 
> Harry looks in the Pensieve. Snape puts his worst memory of 
> James where  it couldn't  possibly color his thoughts. And what 
> did Harry do? Went and fetched it out again. There's no way, 
> now, that Snape can escape it, because now it's part of Harry's 
> memories, too. 

Annemehr:
D at mn, you're good. Why did I never think of that? Unless Snape taught
Harry to put it in the pensieve too (riiight).

Pippin:
> 
> That, plus re-read what happens in the lesson in chapter 26. 
> Just after Harry makes his most successful effort at repelling 
> Snape, just after he breaks into Snape's mind, Snape tries 
> legilimens again--and Harry has a vision *he's never had 
> before*!!!! That's no memory--that's Voldemort, in real time, 
> manipulating Harry's mind right in the middle of Snapes !@#$% 
> office. And Snape knows it, because Harry tells him he's never 
> seen that before. No wonder Snape tells Harry he isn't working 
> hard enough. And Snape is right--Harry made no effort at all to 
> block the vision.

Annemehr:
No, that was a direct result of Occlumency lessons foreseen by
Dumbledore, which is why he felt he could not teach it himself. In his
talk with Harry in his office, he said "I was sure, at the time, that
nothing could have been more dangerous than to open your mind even
further to Voldemort while in my presence." Of course, Harry didn't
practice -- enough -- either. But if he wasn't working during that
lesson, how did he block Snape just before?

Annemehr