It's called escaping, and almost every protocol has it. HN must convert the & symbol to & for displaying in HTML. Many wire protocols like SATA or Ethernet must insert a 1 after a certain number of consecutive 0s to maintain electrical balance. Don't remember which ones — don't quote me that it's SATA and Ethernet.

Protocols that literally insert a bit are HDLC / PPP / CAN and they insert a 0 after a few 1s