[detaro]

e.g. in UTF-8 a codepoint is encoded in varying byte lengths (so you have to split into codepoints and then reverse), and, a lot more difficult, a sequence of multiple codepoints can be combined to form a symbol. Simplest case would be something like "ö" encoded as "o" (U+006F) followed by a combining diaeresis (U+0308).

Other fun special cases: 🇺🇸 is U+1F1FA REGIONAL INDICATOR SYMBOL LETTER U, followed by U+1F1F8 REGIONAL INDICATOR SYMBOL LETTER S and should if possible be displayed as a US flag (otherwise falls back to text "US"), should reversing it create 🇸🇺 (replacing the flag with the characters "SU"), or still show the flag? (I'm not even sure if there isn't a case where both are valid country codes and it would change to a different flag?)

Similarly, Emoji can be formed from a sequence with combining characters inbetween, which don't display correctly if reversed codepoint by codepoint.