In the process of editing a file encoded as UTF-8 w/o [spurious] BOM the content might become devoid of any Unicode characters outside the ASCII or ANSI ranges. At the next reopening of the file, some text editors (Notepad++) will interpret it as ASCII/ANSI encoded and open it as such. Unaware of the change the user will continue editing, now adding non-ANSI Unicode characters, rendered however useless, since saved in ANSI. A menu option can exist (Notepad++) to open ANSI files as UTF-8 w/o BOM, but leading to the reverse issue of inadvertently overriding ANSI files with Unicode encoding.
I recently came across an encoding issue specific to how Firefox encodes URLs directly entered into the address bar. It basically looks like the default Firefox character encoding for URLs is NOT UTF-...