Hi, 

If comint-use-prompt-regexp is t typing in comint buffers inherits text
properties from the prompt. See this screencast: http://screencast.com/t/xYa9KVWu7 

You can also reproduce it by setting comint-use-prompt-regexp to t in
shell buffers. 

Attached is the fix with minor reformatting of the adjacent code for the
readability reasons.

This was probably recently introduced when eliminating the overlays from
comint buffers.
   

Vitalie