MySQL's utf8 charset only supports up to 3-byte symbols. Insterting four byte symbols (U+010000 to U+10FFFF) can be done maliciously to break HTML mark-up.
The ideal solution was to convert to MySQL's utf8mb4 charset, but then we would lose support for MySQL < 5.5.3. In this fix, incompatible characters are encoded as HTML numeric character references (eg. #65536) and just stripped from body_nommarkup.