It's not so fun when you just want to type plaintext but random clients absolutely want to imagine it's markdown and completely butcher your note. I'd be mostly fine with it if they'd stick to only modifying the style and not the text itself. Cf. #[2]
Please do not accept invalid events in your client. Do not accept non-integer "created_at" values, do not accept non-string-array on "tags", do not accept bech32-encoded keys as tag values. Every time you do that, Nostr dies a little.
For more information on how this kills Nostr, read https://fiatjaf.com/27598e6f.html
Discussion
Didn't think about it that way before, but does it mess with #hashtags ?