I get many people wrong by group, often, so yes, all the time. The only tag that I guarantee is completely 100% right at the moment is the INFJs.
I don’t really find that having one rock solid person helps, because there’s so much variation within a type – usually the people I’m unclear on are different enough from the people I’m sure about that I’m mostly going on Te by the time I post them (ie. are they an E or an I, an F or a T, etc.). What does help is finding enough people of one variation that it becomes clear that they are all definitely one type and not another. Also lots of review and watching a pile of video in a row by tag, which makes it easier for Fi to get a sense of the type as a whole and pick out which characteristics are type-related and which aren’t.
So… quantity is key, pretty much.
In terms of people I’m rock solid on… I’ve already kind of refused to do this once – I’m not very comfortable pointing to people as the ‘most’ their type. Also who knows, I’m probably wrong about some of them.
I guess I’ll keep it personal: I just typed the INFP most like me, and that is Benjamin Hochman if he were neither male nor Israeli and were somewhat more introverted. enfjpuppettheatre is like if Miss Irenie Rose grew up to be Sasha Cohen, and infj-zen is most like Jessica Stern. I think I mentioned on her post that JK Rowling is pretty much my mother.
Out of all of these, I think maybe having infj-zen and Jessica Stern as examples helped to fix up the INFJ tag when I got tempted to fill it with other types – the INFJ tag is probably the least celebrity filled tag, though. The rest of us are not especially like a lot of the rest of the people in our tags – or not enough for me to pick up on instantly with Si and map onto other people.