Select Input Method International Coding Systems
When multibyte characters are enabled, character codes 0240 (octal) through 0377 (octal) are not really legitimate in the buffer. The valid non-ASCII printing characters have codes that start from 0400.
If you type a self-inserting character in the invalid range 0240 through 0377, Emacs assumes you intended to use one of the ISO Latin-n character sets, and converts it to the Emacs code representing that Latin-n character. You select which ISO Latin character set to use through your choice of language environment (see Language Environments). If you do not specify a choice, the default is Latin-1.
The same thing happens when you use C-q to enter an octal code in this range.Select Input Method International Coding Systems