Readline
In many terminal programs readline is doing the editing of input. It is possible with M-v to quickly switch to vi mode and with M-e to the default emacs mode. In emacs mode one has a few commands like ^A to jump to the begin of line. Usually I prefer vi - but in command line it’s not that useful to switch modes.
R in tmux got stuck in vi mode and was pasting ^A^K... all over the place.