Harry is viewing his father tormenting Snape in the Penseive. Snape comes in and grabs Harry’s head out of the waters. The first thing anyone says about which vision Harry saw was Snape’s “Amusing man your father, wasn’t he?” But how could Snape have known what Harry was seeing? There are several problems here.
The text says that Snape removes several memories from his head before occulmency classes. We know that the memory Harry saw was one of them, but there must have been others: especially everything relating to Snape’s work both past and present with Voldemort (things like him being the bearer of the prophecy to Voldy as well as colluding with him after GoF, whether this collusion is as a true Dumbledore-loyal spy or as a double agent). Snape wouldn’t want Harry to see any of that stuff.
Now, given that Snape has removed these memories from his head, we have another problem: he shouldn’t remember what they were about or what happened in them! He probably shouldn’t even remember what the memories were about.
The only thing I can think of is that Snape could see the vision in the waters around Harry’s head, though given what we know of the size of the Pensieve and how fast Snape would want to pull Harry out once he walks into the room.
But given how much Snape turns out to have to hide, you have to wonder whether Snape would worry that Harry had seen more than just his father. In fact, it’s bizarre in retrospect that Snape lets Harry go so easily without finding out exactly what secrets have been spilled.
Snape’s specialty is reading and hiding thoughts. It’s very possible he somehow masked the thoughts he didn’t want read (the Death Eater years, childhood, etc.) in such a way that Harry couldn’t possibly view them, but deliberately left the negative memories of James Potter readable. I think he wanted Harry to know what a snot his father was and why Snape hated him.
I don’t know the whole theory behind the pensieve, bup, but this would be my explanation:
Before the class, Snape puts memories in the pensieve that he doesn’t want Harry to see.
Snape leaves the room, comes back, and Harry is gone. He fears that Harry is in the pensieve, and Snape knows that whatever memory is in the pensieve (even though he can’t remember what exactly it is) is something he doesn’t want Harry to see. And in any case he would be upset and Harry doing something he wasn’t supposed to do.
He dives in there after Harry, and then sees that Harry is witnessing Snape’s humiliation at the hands of James Potter.
Ah, you are right: Snape is there IN the vision. But still, Snape should probably fear Harry of having seen a lot more than just the vision he was looking at when Snape came in. But at least in OoTP, Snape only seems worried about the James vision.
The text does say that Snape removes the memories so that Harry won’t see them in case Harry accidentally breaks through Snape’s mind. It’s pretty certain that the darker secrets must be there.
If Snape is worried, then it seems like legilimency must to work by connecting minds: even if you are a very powerful Legilimens, there is a danger that while reading someone else, they’ll get flashes of yourself as well, overcoming your own occulmency. Harry is so pathetic at both that I’d find it hard to explain why Snape would be so worried as to use the Pensieve otherwise.
Seems a bit like the concept of “cloaking” in Star Trek where you have to lower it in order to fire. Except, of course, in Star Trek 6, where a “fire-while-cloaked” technology is used that the universe then apparently completely forgets about from then on.
The cop-out answer, of course, is that for her plot J.K. Rowling only needed Harry to see that one memory, and it would have slowed down the scene too much to have Snape start babbling about othter memories hidden in there.
But how does Snape know that Harry only saw one memory? I could explain it in several ways:
From the length of time Snape was gone, he would know that Harry only had time to see that one memory;
Snape, being (I assume) a good legilimens in addition to being a master of occlumency, looked in Harry’s mind while Harry was in the pensieve, or, more probably, after he pulls Harry out of the pensieve, and realized that Harry only saw the one thing.
Why would Snape need to ask? We’ve already seen that, inlike Voldemort but like Dumbledore, he pretty much has easy access to Harry’s thoughts - Harry’s Occlumency training was to cover his poor mental control, but in HBP Snape quite clearly uses legilimency on Harry several times, one time in fact to judge whether Harry’s telling the truth (about the book). Perhaps after Snape pulled him out, he just checked Harry’s mind for any hint of him hiding something, and found nothing.
I have always believed that the only memories in there were bad memories of James and Sirius, probably all similar in nature to the one Harry saw. Far from being memories Snape didn’t want Harry to see, I think those were memories Snape picked specifically for Harry’s viewing pleasure. So, “amusing man, your Father”, would have applied to any of the memories.
Snape is very precise and careful. If he really didn’t want Harry to see those memories he would have put the penseive away in a cupboard - probably a locked one.
Snape did not believe Harry could do it. Snape’s worst memories probably never left his mind as that was the safest place for them.
Snape knows Harry is far too nosey not to look if given the opportunity. (if the opportunity had not arisen, I’m sure Snape would have made some excuse to leave Harry alone).
I’ve always been sure that Harry did exactly what Snape wanted him to do.
The problem is that legilimancy has some pretty obvious effects on the victim: Harry feels Snape rifling through his mind in HBP.
I also don’t buy that Snape wanted Harry to see the memory. If he did, why does he banish Harry from his office forever and demand that he never speak of what he saw? All of which is against the direct orders of Dumbledore.
I have no reason to think that Rowling’s stuff about how Snape was putting things into the Pensieve “in case Harry broke through” is anything but straightforward. Sure, it’s Harry thinking it, but there’s no hint that he’s wrong or mistaken about why the memories are going in. Heck, Dumbledore KNOWS about the use of Pensieve for goodness sakes.
This, I think, is one of the biggest “issues” with the pensieve.
When Rowling first presents the pensieve (in the Goblet of Fire, where Harry sees the trials of the Deatheaters), Dumbledore tells Harry that it is primarily a way of examining old memories more closely. Even there, though, it seems obvious that Dumbledore is not only aware of the memories that Harry has accidentally seen, but that he still has conscious awareness of those memories, even after transferring them to the pensieve, since he is able to converse with Harry about those memories.
In OOP, however, Snape is supposed to be using the pensieve as a back-up for memories that he doesn’t want Harry to know about, with the idea that if the memory is transferred to the pensieve, it is no longer available to the original person.
Then, when you get to Half-Blood Prince, the pensieve becomes primarily a means of viewing other peoples’ memories. One of those memories is a distorted memory, and Harry is asked to retrieve the original memory. However, if the memory is completely removed from the person before it is transferred to the pensieve, then there should not be any way to get another copy of the memory.
My own feeling is that you can either cut or copy memories to the pensieve. Snape probably “cut” the memories, but if that’s the case, why would he even bother to put them back later, since they are such painful memories.
Nobody has mentioned this yet: we don’t know how long Snape was in there for. Harry was watching his dad taunt teen-Severus and then Snape grabbed him and said pulled him out. He could have been waiting until it got to the point where James was about to take off his (Snape’s) underpants and then took Harry out. It’s been said about how Snape might have wanted Harry to see what a jerk his dad was so that’s why he waited until it would have gotten awkward…