OK, I went back and read the original post and the question is:
What do we think the dangers are if a reading has been tampered with? So that really wasn't addressed, I don't think. We all thought it was more about whether it was ethical to do a tampered reading. That's what I thought, anyway.
So what do I think the dangers would be? Without knowing more precise facts, it's hard to pinpoint what the dangers would be in a specific situation. The dangers, speaking in general, would probably take a whole page to list. If I knew a specific scenario it would be easier, but I'll give it a shot.
One of the dangers---a very big one---would be that the sitter would feel incredibly betrayed if she ever found out. Very often our sitters come to us for guidance about things they feel very strongly about and that are vital to them. They depend on us and trust us with their lives.
Another danger, which might sound a little far-fetched but it's not, is that we'd have breached our own ethics and that would make it easier to allow the tampering the next time someone wanted to tamper. And then once it had been done a few times, it would be easy to let a little cash exchange hands to do a bigger and bigger tampering job. And then, you wouldn't be fit to be trusted to read grafiti on a bathroom wall, in my opinion.
Then there's the danger that you're the only person left in the world that the sitter trusts and if not for trusting you, she'd just give up and walk in front of a train. Except that often people who are suicidal are in danger of taking someone with them if they're going to commit suicide and that could end up being you.
And then there's the danger that you might be contributing to keeping an abused woman in the clutches of her abuser, when if you hadn't allowed the tampering she might have actually gotten the information she needed to have the courage to get out of the relationship.
There are so many things---there's not even room in my imagination to think of good examples.
Another real big one that comes to mind is that you might get arrested.