programs on a system with the setting en_US.UTF-8
and acting accordingly will process Ñ ñ encoded as \xc3\x91 \xc3\xb1 correctly and without any complaint. If the program is led to believe that the data is encoded according to ISO-8859-1, not
much would happen except that a single Ñ or ñ would result in two characters. If, however, Ñ ñ are encoded according to ISO-8859-1 as \xd1 and \xf1, a program following en_US.UTF-8
will have to indicate an error since no UTF-8 encoding (a multibyte encoding) begins with either characters.
Using /usr/bin/od to look at the "raw" data is a useful first step to see what is going on.