A byte-order mark — an invisible sign at the start of a file saying how it is encoded. Small, unseen, and capable of breaking everything downstream.
Initials for 'byte order mark': byte a deliberate 1956 coinage (Werner Buchholz, IBM, respelling 'bite' to avoid confusion with 'bit'), order from Latin ordo, mark from Old English mearc (a boundary sign); together, the small unseen sign a file opens with to declare which order its bytes run in.
An invisible sign at the very start of a file, small enough to miss and large enough to break everything that reads the file afterward if it is misjudged.