My testing shows that if I hit both
Enter and backslash keys at almost the same time, this can be reproduced. Assuming the keyboard is well "debounced" (link to info about debouncing) then there was only a single backslash typed so in fact there is a "spurious second echo".
The (single) backslash key can be "deleted" via backspace but only one of the two echos will disappear. The echo to the right of the green prompt will disappear when backspace is keyed. The echo to the left of the green prompt cannot be removed. Apparently I did not "delete" it and proceeded to use the
Enter key again. The
backslash Enter pair is a line continuation sequence which works inside scripts but apparently also interactively. As mentioned by @IgnacioVazquez-Abrams the prompt becomes the one assigned to continued lines and in my case it happens to have a different appearance that is not the same as my usual green prompt.
In my testing, if backspace is used to "delete" the backslash (even though only one of the two echos will be removed) there is no line continuation. This is demonstrated by using the backspace to "delete" the backslash and then hitting the
Enter key. In my test the prompt remained green which means that there was no line continuation sequence formed.
mv command was not affected by the backslash because the
Enter key was pressed slightly before the backslash. Proof of this is that the backslash appears on a new line (albeit before the green prompt) which means
Enter, which echos as a newline, apparently was pressed first. Since the
mv command was not affected by the backslash there will be no unexpected files created.
I believe I have seen similar behaviour when using
ssh and I am typing fast. The remote host cannot prompt for the password soon enough so part of my password gets echoed to the left of the password prompt and may be echoed to the right of it also. I don't remember whether the password is obfuscated in both places or only on the right of the prompt.