Surprisingly many people try to abuse Unicode when posting submissions and comments. This includes not just emojis, but also other symbols, or glyphs that look like bold, underlined or otherwise stylized letters.
You can play mahjong, domino or cards (🀕, 🁓, 🃅) but not chess, you can show some random glyphs like ⏱ or ⤪ but not others, you can use box-drawing characters (┐) but not block elements, you can use byzantine musical notation (𝈙) but only western musical symbols (𝅗𝅥) and the notes are banned, you can Z̸̠̽a̷͍̟̱͔͛͘̚ĺ̸͎̌̄̌g̷͓͈̗̓͌̏̉o̴̢̺̹̕ just fine.
