Sure happy to define it.
- Person A, Person B and Person C are all “up to date” (seeing the same most recent message).
- The history of Person A can contain one or more messages that are not in the history of Person B *in any form*, not a deleted-message tombstone, nothing, no trace whatsoever. And the reverse. And the same for Person A-C, B-C.
- Additionally the history of Person A can be missing messages that appear in the history of Person B, again with no tombstone, no trace, no indication whatsoever that something is missing.
-These "He sees it, she doesn't" messages can be sent simultaneously, so at the same time (and with a malicious client) Person D can send a separate message to Person A, a separate message to Person B, and a separate message to Person C. (or send to all but A at the same time).
Also I'd add that any detection method cannot result in a false positive and is therefore explicitly trusted.
The above combination of factors enables a level of social engineering that you can't really compare with what an attacker could achieve on Signal, WhatsApp, Telegram, etc. Yes of course people will get scammed anyway. Scammers love Telegram, and Telegram's serer ensures the above can't happen. What I'm arguing is that after getting scammed on Telegram it's fair to say (if cruel) that they have themselves to blame. For this case, if you game theory it out, I don't think we can say that they do fully have themselves to blame. It's a very unique attack vector.