Here are some cases of mass 'hacking' of Telegram. The problem with Telegram is these 'hacks' give attackers access to entire Telegram chat histories, unlike E2E apps like Signal, WhatsApp, Wire etc.

[1] Brazil politicians' Telegram 'hack' of 2019

https://www.wired.com/story/brazil-hacker-bolsonaro-car-wash...

[2] Iran Telegram 'hack' of 2016

https://www.reuters.com/article/us-iran-cyber-telegram-exclu...

[3] Israeli cryptocurrency executives' 'hacked' on 2020 including their Telegram

https://www.haaretz.com/israel-news/tech-news/.premium-exclu...

[4] Moxie Marlinspike of Signal app on Telegram

https://twitter.com/moxie/status/1474067549574688768

Hack is in quotes because all involve SMS interception.