Without knowing the details of C, I've seen this in other languages and it's usually something with missing a flush or a buffered output mode or something like that.

Im pretty sure its because of char 13 (carriage return). This char sets cursor to the start of the line overwriting whatever was printed there (in most terminals). I belive that some error messages use this char and when you print something the char at the begining or end of the error message overwrites your message. A workaround is simply printing a newline after or before your message.