A lot of software predates UTF-8. For example, scripts on Unix-like (Unix, Linux, BSD) systems. In order to determine that a file should be executed using an i

A lot of software predates UTF-8. For example, scripts on Unix-like (Unix, Linux, BSD) systems. In order to determine that a file should be executed using an interpreter, they look for the ASCII bytes "#!" at the beginning of the file. If those bytes are not present (such as if the file is UTF-8 with a BOM), then it won't be executed with an appropriate interpreter. Now, once the interpreter is found, it can interpret the rest of the text as it sees fit (for instance, interpreting it as proper UTF-8). But the first two characters of the file must be "#!". Furthermore, the BOM is simply a bad idea. It was poorly conceived from the beginning. It means something different at the beginning of a file than it does in the middle (at the beginning, it is the BOM; in the middle, it is a zero-width non-breaking space. This means that you can't simply append files or strings that begin with the BOM, and have the result still be valid. The BOM should not be used for sniffing files, except as a fallback. Sniffing is a terrible way to tell what encoding a file is in. The format should be recorded somewhere (in file metadata, by default, by the locale, or something of the sort). Sniffing should only be used as a last resort.