And it's a very bad practice. No experiment ever should be taken without an assistant (sitter in this case).

Bad advice. Yes there should be at least one other person who knows what you're doing, but if you're experienced in the matter, solitude is the best practice. The emergency contact person could be a phone call away, just in case.

problem is that when you are really in deep, you are not able to make that phone call. It is good advice to always take someone with you. It is like saying: more experienced bikers don't need to wear helmet. Yes, 99.99% of the time things will be fine, but it's still good advice to always wear one. Because that 0.01% can lead to severe consequences.

Exactly.

Unexpected things happen, mental states swing, and now you're in a mentally compromised state in-between reality.

"I know what I'm doing" said the victim before they died.

Why do you say that? I generally just sit there alone, listening to music and observing inner processes.

There are some other commenters on here that seem to implicitly be talking about what I'd consider hero doses. I don't need high doses to experience notable differences in my ability to sense and self-reflect.

Ehh, I would qualify that with it mattering how much you are going to take. Like if someone was going to down an entire bottle of vodka and get raging drunk they should probably have someone around to watch them too, but if someone drinks a single beer it would be a huge over reaction to consider it too dangerous to do alone.