readline version: 6.3_p8 and the one in devel branch of bash
OS: gentoo linux "unstable", kernel 3.17, UTF-8 locale
To reproduce:
in bash, press Ctrl-R to invoke reverse-i-search mode, enter any multi-byte character (on US keyboard layout LAlt+1 seems to produce "±" which is multi byte), press backspace.
Expected behaviour: last character being erased. Actual behaviour: apparently a single byte is erased instead, leaving an incorrect UTF-8 character.