The following script should print the first line read from the file but instead it produces the error, "invalid or incomplete multibyte or wide character". There is a utf-8 encoding in the second line, not the first:

set chan [file tempfile]
chan configure $chan -encoding binary
# \xc0 is invalid in utf-8
puts -nonewline $chan a\nb\xc0\nc\n
flush $chan
seek $chan 0
chan configure $chan -encoding utf-8 -buffering none -eofchar {} -translation lf -profile strict
puts [gets $chan]

Fixed in [67baae2829].