The handler for the "M" key in the AutoResponder needs to suppress the keypress. Right now, if you have a rule in the "If request matches" column that starts with "M" and you select a different rule, hit "M" to add a comment to it, then Windows automatically focuses the list item that starts with "M", then Fiddler invokes the "Add Comment" action on that rule instead of the one originally selected.
This bug still exists almost 18 months later. :'-(
The trick here is actually to move the actSetRuleComment call to the lv_RespondRules_KeyUp handler instead of putting it in the _KeyDown handler. This problem also exists in the Composer History listview.