Pensieve can't be cheated
I have said several times that I will tell everyone my thoughts why it cannot be faked, so here I go, as I am leaving this forum most probably.

An interview by Rowling:

Q: Do the memories stored in a Pensieve reflect reality or the views of the person they belong to?
A: It's reality. It's important that I have got that across, because Slughorn gave Dumbledore this pathetic cut-and-paste memory. He didn't want to give the real thing, and he very obviously patched it up and cobbled it together. So, what you remember is accurate in the Pensieve.

Q: So there are things in there that you haven't noticed personally, but you can go and see yourself?
A: Yes, and that's the magic of the Pensieve, that's what brings it alive.

Q: I want one of those!
A: Yeah. Otherwise it really would just be like a diary, wouldn't it? Confined to what you remember. But the Pensieve recreates a moment for you, so you could go into your own memory and relive things that you didn't notice at the time. It's somewhere in your head, which I'm sure it is, in all of our brains. I'm sure if you could access it, things that you don't know you remember are all in there somewhere.

Now I don't count interviews as canon, but Rowling says that it's important that it got across, that is that readers can guess by reading the books. I could, I found this interview after I had come up with my theory.

Look at the first time we meet pensieve, Harry can see Dumbledore himself. and everyone else too doing things without any breaks or jerks or pauses. That's not normal memory, in a normal memory Dumbledore or anyone couldn't have noticed such things, he definitely couldn't have seen himself from a third person's perspective. This is not mere memory or thoughts. If they were, we would have seen everything first perspective, even if you assume perfect recall which pensieve shows, we will still be in first perspective only and seeing what Dumbledore saw. Harry on the other hand could mve and see other people and their reactions, he never noticed that they felt incomplete or unnatural which would have been the case if it was mere memory as Dumbledore couldn't have seen all of that, even if perfect recall is there, Harry can see things Dumbledore definitely didn't see because he was not looking at those things. More than memory.

Another point taken from wiki, Most interestingly, the memories viewed by the person watching in the Pensieve are more complete than the person's own observations. For instance, in Harry Potter and the Half-Blood Prince, Bob Ogden visits the Gaunt family. Morfin speaks to him only in Parseltongue, which Ogden does not know and so would not be able to remember properly; yet Harry, reliving Ogden's memories, not only understands what the Gaunts are saying in Parseltongue, he is able to perceive things happening outside Ogden's range of vision.

I can give examples from every time Harry goes into pensieve.

From book and Rowling's mouth is evidence that this is not memory.

Now cheating pensieve. You cannot dream or think of a different scenario and give that memory, as the person's thoughts are not heard or seen in case of visual thoughts in the pensieve. We don't see what's going inside Dumbledore's head, Throw away your counter argument of dreaming of a different scenario.

Now changing things as they appear in the pensieve. You cannot cast a spell on the people in the memory, can you? No. So how will you make them do something else or say something else? The most you can do is make everything hazy and record over, which Slughorn does, that is easily noticeable. In fact, everything goes white, or you can read lips, and audio won't be in sync. Video cannot be altered.

Description of Slughorn's memory:

"Harry noticed that the contents proved difficult to empty into the Pensieve, as though they had congealed slightly; did memories go bad?"

Now make whatever you want of this, call me names, blah blah, I don't care.

To Taure-

He didn't alter at all what the pensieve showed, read:

As several of the boys tittered, something very odd happened. The whole room was suddenly filled with a thick white fog, so that Harry could see nothing but the face of Dumbledore, who was standing beside him. Then Slughorn's voice rang out through the mist, unnaturally loudly, "You'll go wrong, boy, mark my words. "

The fog cleared as suddenly as it had appeared and yet nobody made any allusion to it, nor did anybody look as though anything unusual had just happened. Bewildered, Harry looked around as a small golden clock standing upon Slughorn's desk chimed eleven o'clock.

So the part where it was altered, the pensieve showed nothing, and the voice was was "unnatural" too. Even if you make the voice perfect, you cannot change what the pensieve shows, as the person whose memory it is, has absolutely no control over what the pensieve shows. It shows everything, things you didn't even see.

 

Why do you think this one is going to go down better than the last one?