set f [open $fn w+]
    fconfigure $f -encoding binary
    # In UTF-8, a byte 0xCx starts a multibyte sequence and must be followed
    # by a byte > 0x7F. This is violated to get an invalid sequence.
    puts -nonewline $f "A\xC0\x40"
    flush $f
    seek $f 0
    fconfigure $f -encoding utf-8 -buffering none
} -constraints knownBug -body {
    set d [read $f]
    binary scan $d H* hd
    set hd
} -cleanup {
    close $f
    removeFile io-75.6
} -result "41"

    fconfigure $f -encoding binary
    # In shiftjis, \x81 starts a two-byte sequence.
    # But 2nd byte \xFF is not allowed
    puts -nonewline $f "A\x81\xFFA"
    flush $f
    seek $f 0
    fconfigure $f -encoding shiftjis -buffering none -eofchar "" -translation lf
} -constraints knownBug -body {
    set d [read $f]
    binary scan $d H* hd
    set hd
} -cleanup {
    close $f
    removeFile io-75.9
} -result "41"