Probably some kids nickname in style of "xXxK1ll3RxXx" is so cringy that even console can't take it any longer.

[̪͝ḭ̮͢n̹̦̺͍͜v͓̻̘͇͈̞o̧k̞͔͎̼̲̦e̛͈̬̫̜ ̡̲t̹̥͕͚̘͟h̖̪̲̥͞e̷̯͙̲̬̘ ̵̥̲hi̯͙̙̝v̟͇e҉-̀m̪̩i̭͉̰̘̼̣͜ń̜͍d̠̬̞͔̰͇͞ ]$$_420_B00+y_kILL3R_420_$$ Edit: I remember there was similar issue in steam chat.

Issue is not that big and doesn't brick consoles, PS4 can be easily recover in a couple of minutes by rebuilding database. This will be fixed in upcoming FW update but in the meantime to prevent the possibility of this happening you can block receiving messages from anyone in privacy settings. More info here: https://www.reddit.com/r/PS4/comments/9o5efg/message_bricking_console_megathread/