1) M-x shell
2) echo foo RET
3) C-/ to undo this 'RET'
   The buffer now contain the shell's prompt and "echo foo".
   However, the process mark is located at eob after "echo foo"
4) Type bar
   The buffer now contain the shell's prompt and "echo foobar"
5) RET
   Shell will output "bar: command not found", because the process mark
   is located before "bar" after "foo".

Similar behaviour can be observed with C-c SPC (comint-accumulate) and
with eshell.

My idea to solve this is to record process mark and related marker
positions as `apply' entries in the undo list. Attached patch implements
this for comint and eshell.