I get a bunch of messages from my friend on Steam, so I tap to read them, but if I tap the wrong way it opens the app instead, makes all the messages disappear, and I have to wait until I get to a place with internet again before I can see what he said.
Another thing that drives me absolutely nuts in iOS, and that stems form its lack of a usable notification system, is that it considers it okay to interrupt your train of thought with modal alerts. When your battery gets low, you get an alert in your face. When there's a system update, you get an alert in your face when you unlock the device (and then an annoying, very dark-pattern-y "enter your passcode to install overnight" screen). When there's any number of things about Apple services you couldn't care less about, you get an alert in your face, too. On (stock) Android, all these things are notifications that can be ignored for as long as you desire.
Discord is the same way and it drives me mad, especially considering how fussy it can be about connecting. Snapchat on the other hand handles it right. If I get a notification (for an actual chat at least), I am able to open it anytime later regardless of whether or not my phone is still connected.