Developer says, "hey we've got a great idea, but we've got to rewrite everything to make it work". Product manager says, "ok what will it do.". Developer says, "oh, you can have a little blinky light or something on the UI". Product manager says, "great but it seriously takes all that time just to do that"?
If your working with a really seasoned developer with good people skills that's when they'll take you out for a beer. :D
Unless you guys are working agile / scrum. Then no one gets beer because the scrums end up being too early in the morning. Instead replace the word "beer" with "coffee". :D