I started emacs from xterm (via a ssh session) on some text file and
quitted it with C-x C-c almost immediately. As a result, I got

^[]11;rgb:0000/0000/0000^[\%

(the % is from zsh, because of the lack of \n), and
"11;rgb:0000/0000/0000" appeared in the command line.

I suspect that this is data sent by xterm as a response from some
escape sequence sent by emacs. This could mean that emacs quits
before waiting for the answer from the terminal.

I can't reproduce this issue, but I think that a temporary network
slowness (remember, this is via ssh) may trigger it.