Dumbledore says: "It is, as you will have noticed, very crudely done", which implies that it would have been possible to have done it much better.
Not long before this, they had looked at a memory of Morfin Gaunt, and we find out that Voldemort had implanted a false memory and Dumbledore had to go to some trouble to get the real one: "...performed the complex but of magic that would implant a false memory in his uncle's mind [...] a great deal of skilled Legilimency to coax [the real memory] out of him."
That false memory was only described/confessed to by Morfin, though, as far as I can tell, and not pulled out and looked at in a Pensieve. Certainly its appearance isn't described for us, if it was.
Perhaps it's a difference in voluntary self-modification vs. involuntary modification by someone else. Slughorn seems to be perfectly aware of his real memory, whereas Morfin's real memory was repressed.
But in any case, it would seem that memory-tampering is not always obvious by nature.
no subject
Not long before this, they had looked at a memory of Morfin Gaunt, and we find out that Voldemort had implanted a false memory and Dumbledore had to go to some trouble to get the real one: "...performed the complex but of magic that would implant a false memory in his uncle's mind [...] a great deal of skilled Legilimency to coax [the real memory] out of him."
That false memory was only described/confessed to by Morfin, though, as far as I can tell, and not pulled out and looked at in a Pensieve. Certainly its appearance isn't described for us, if it was.
Perhaps it's a difference in voluntary self-modification vs. involuntary modification by someone else. Slughorn seems to be perfectly aware of his real memory, whereas Morfin's real memory was repressed.
But in any case, it would seem that memory-tampering is not always obvious by nature.