Is stream entry an unmistakeable experience? I've heard stream entry described as a turning about in the deepest seat of consciousness. I've also read other accounts that make me think that there is something about the experience that is unmistakable. Is that true? If one were to attain stream entry would one know it without doubt. Is it such a notable experience that it couldn't be mistaken for anything else?
Tibetan Buddhism recognizes two kinds of milestones: "experiences" and "realizations".

Experiences are all kinds of extraordinary moments one has along the way. Like the experience of non-duality: "I am what they call God, the true goal of all religions and the real meaning of love". Experiences come and go. They leave memories, but they don't stay with you in all their might. And that's probably a good thing. I don't know how I would live walking around thinking I'm God ;))

Realizations are (sudden or gradual) breakthroughs that change one's perspective in fundamental ways. Needless to say, they stay with you, although not necessarily in an explicit form, because you tend to get used to them and stop noticing. An interesting feature of realizations, they sometimes can be retrograde (like retrograde amnesia). It slowly creeps on you or dawns on you, that you have known something or have been behaving differently for a while now. It is almost like realization spreads backward in time.

By this classification, stream entry is a realization, not an experience. Because of its nature, in most cases I suspect it is a retrograde realization.

The true stream entry is unmistakable, yes -- but I don't see why someone could not just take some arbitrary experience, or even some intermediate realization, like the realization of emptiness of so-called "objects", and imagine that to be stream-entry.

What makes it unmistakable is the fact that in order to attain it, you have to run out of other options. You have to exhaust all theories, all hypotheses, all alternative routes to enlightenment. That's why you don't hesitate, because you clearly see that there is no way it could be something else.

What's funny is this nagging feeling I have, that you reading my description of it, reduces your chances of attaining it. Enlightenment is freedom from clinging (among other things), so clinging to this notion of stream-entry must be very counterproductive. This is why I'm a little leery of Theravada's sometimes excessive emphasis on bullet-point lists and precise definitions of the stages, as well as of Zen's obsession with the notion of satori. The more your mind grasps these, the further you are from the goal.