Here's a simple patch to tweak the behavior of
(comint-get-old-input-default) to do what the docstring says it should
do. Example:

1. emacs -Q
2. M-x shell
3. seq 5 [RET]
   This produces the output

   1
   2
   3
   4
   5

4. Move point to "3"
5. (comint-get-old-input-default)

According to the docstring, this should return the current line; i.e.
"3". Instead it returns "3\n4\n5\n" + the next prompt. This behavior can
produce lots of unwanted commands executing accidentally, and it goes
against what the docstring says.